Here's one that's not so obvious:

Solve 

(Harder to guess the answer now, huh?)

Get the x alone...  Who's bugging him?  The 3!

Since the 3 is multiplied with the x, we'll undo him by dividing both sides by 3:

Hey -- Did you know that you can check these things?

TRY IT:

Solve        *And check it!