Always remember PEMDAS:
P- parenthesis
E- exponents
M- multiply
D- divide
A- add
S- subtract
Remember you do what's in parenthesis first before anything else. But if you can't combine like terms inside the parenthesis, you might have to distribute what number is out in front of the parenthesis.
3x+15=3(x+5) First you distribute the 3. You should get 3x+15=3x+15. The 3x will cancel each other out and so will the 15. So you're answer is undefined.