Compilation failed | Execution failed for task SOLVED | ANDROID STUDIO

Error:(18, 13) error: cannot find symbol class RewardedVideoAd
Error:(25, 9) error: cannot find symbol variable MobileAds
Error:(27, 15) error: cannot find symbol variable MobileAds
Error:(37, 78) error: package AdRequest does not exist
Error:Execution failed for task ':app:compileDebugJavaWithJavac'.
Compilation failed; see the compiler error output for details.

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Android Studio
Developer(s) Google, JetBrains
Stable release 3.1.2
Preview release 3.2 Canary 10
Repository
Edit this at Wikidata
Development status Active
Written in Java, Kotlin
Operating system Windows, macOS, Linux[3]
Size 847 MB compressed
Type Integrated development environment (IDE)
License Freeware +Source code
Android Studio is the official integrated development environment (IDE) for Google's Android operating system, built on JetBrains' IntelliJ IDEA software and designed specifically for Android development. It is available for download on Windows, macOS and Linux based operating systems. It is a replacement for the Eclipse Android Development Tools (ADT) as primary IDE for native Android application development.

Android Studio was announced on May 16, 2013 at the Google I/O conference. It was in early access preview stage starting from version 0.1 in May 2013, then entered beta stage starting from version 0.8 which was released in June 2014.The first stable build was released in December 2014, starting from version 1.0. The current stable version is 3.1 released in March 2018.

Contents
1 Features
2 System requirements
2.1 Version 3.x
2.2 Version 2.x
2.3 Version 1.x
3 Version history
4 References
5 External links
Features
The following features are provided in the current stable version:

Gradle-based build support
Android-specific refactoring and quick fixes
Lint tools to catch performance, usability, version compatibility and other problems
ProGuard integration and app-signing capabilities
Template-based wizards to create common Android designs and components
A rich layout editor that allows users to drag-and-drop UI components, option to preview layouts on multiple screen configurations
Support for building Android Wear apps
Built-in support for Google Cloud Platform, enabling integration with Firebase Cloud Messaging (Earlier 'Google Cloud Messaging') and Google App Engine
Android Virtual Device (Emulator) to run and debug apps in the Android studio.
Android Studio supports all the same programming languages of IntelliJ, and PyCharm e.g. Python, and Kotlin; and Android Studio 3.0 supports "Java 7 language features and a subset of Java 8 language features that vary by platform version." External projects backport some Java 9 features.