The import javax.servlet can't be resolved

You need to add the Servlet API to your classpath. In Tomcat7.0, this is in a JAR called servlet-api.jar in Tomcat's lib folder. You can either add a reference to that JAR to the project's classpath, or put a copy of the JAR in your Eclipse project and add it to the classpath from there.
If you want to leave the JAR in Tomcat's lib folder:

• Right-click the project, click Properties.
• Choose Java Build Path.
• Click Add External JARs...
• Browse to find servlet-api.jar and select it.
• Click OK to update the build path.

Or, if you copy the JAR into your project:

• Right-click the project, click Properties.
• Choose Java Build Path.
• Click Add JARs.
• Find servlet-api.jar in your project and select it.
• Click OK to update the build path.