Cannot fit requested classes in a single dex file (# methods: 79313 > 65536)

For resolve this error follow these steps:

Fristly create android project ,type name like MyApplication3 (Maintion in this project)

if you do'nt know how to create android project ,then follow below link

How to create android project in android studio

Type below code in activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:ads="http://schemas.android.com/apk/res-auto"

    tools:context=".MainActivity">
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <com.google.android.gms.ads.AdView
        android:id="@+id/adview"
        android:layout_width="match_parent"
        android:layout_height="63dp"
        ads:adSize="LARGE_BANNER"
        ads:adUnitId="ca-app-pub-6170006774994819/4974882782"
        >

    </com.google.android.gms.ads.AdView>


</RelativeLayout>

Type below code in app level gradle file

 plugins {

    id 'com.android.application'
}

android {
    compileSdkVersion 31
    buildToolsVersion "30.0.3"

    defaultConfig {
        applicationId "com.example.myapplication"
        minSdkVersion 19
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"
        multiDexEnabled=true


        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
}

dependencies {

    implementation 'androidx.appcompat:appcompat:1.4.0'
    implementation 'com.google.android.material:material:1.4.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.2'
    implementation 'com.google.firebase:firebase-ads:20.5.0'
    testImplementation 'junit:junit:'
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    implementation 'com.google.android.gms:play-services-auth:20.0.1'
    apply plugin:'com.google.gms.google-services'

    implementation 'androidx.multidex:multidex:2.0.1'


    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
}

Type below code in project level gradle file

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath "com.android.tools.build:gradle:4.1.3"
        classpath 'com.google.gms:google-services:4.3.10'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

    
    
    
    




    
    Type below code in AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.myapplication" >

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.MyApplication" >
        <activity android:name=".MainActivity" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

Connect android project to firebase 
How to connect android project to firebase in android studio
Download google-services.json file from firebase