How do you solve equations that contain like terms?

All like terms should be transferred to the same side of the equal sign <<<< answer
Let's use an example. It's easier to answer after that.
9x - 33 = 8x + 21

The trick is to get the letters on one side, and the numbers on the other. I think that is likely the answer the question is looking for.

We'll get the x's on the left.
Subtract 8x from both sides.

9x - 8x -  33 = 8x - 8x + 21 The 8xs on the right cancel out.
x - 33 = 21 So I have all the xs on the left hand side (one x) Now add 33 to both sides
x - 33 + 33 = 21 + 33
x = 54.

This may be too advanced a concept. Leave a note if you are still confused or wonder.