The goal of simplifying an equation is to get it to a place where you cannot make the answer any simpler. Here's an example of an answer that is NOT simplified:
7x + 3x + 9 + 6
The reason this is not simplified is that there are multiple terms that can be combined. The two terms with 'x' in them can be combined to form one term and the two constants (numbers without a variable attached) and be combined to form one number. Remember that you CANNOT combine terms with variables with constants by addition or subtraction, only multiplication and division. So 6(2x) can be simplified to 12x but 6 + 2x cannot be simplified any further.
With all of that in mind, look at the terms 15 and 10x and ask yourself "can I combine these two terms?" If the answer is yes, then combine them. If the answer is no, then you've arrived at your answer.