You'll have to start off by finding the variables. Ex:
3(x + 3) - 2(x + 3)
As you can see here, the two similar variables are x. By using the distributive property, you will get this:
3x + 9 - 2x + 6
Next, you'll want to subtract the two variables, since 2x has a - in front of it:
1x + 9 + 6
or
x + 9 + 6
Then, you want to add the two numbers without the variable:
x + 15
Your answer is x + 15.
Hopefully that helped! :)