Car with rotating wheels step by step

This is a slightly different way, hopefully easier to understand.

Create a new document.
On the stage draw a wheel using the oval tool. Make a few spokes with the line tool in order to be able to see the rotation.

Select the wheel and convert it into a graphic symbol (under the modify menu)

Under the insert menu select new symbol. Make it a movie clip.

This will put you inside the symbol. Notice the mc icon next to the scene 1.
Drag the wheel from the library(the graphic you made before) to the empty white space. Now you have a graphic symbol inside a movie clip.

On the timeline of the movie clip (make sure you are still inside it) animate the graphic applying a rotation.

Next click on scene 1 under the timeline. This will bring you to the main timeline.

If the graphic is still there get rid of it. Then pull from the library two instances of the movie clip and set them side by side as wheels. Draw a body for your car.

Select both wheels and the car body and turn them into a graphic symbol called car.

Animate the car on the main timeline.