Answer
x + 7.
Explanation:
The answer is X + 7 because you know, 8x + 13 and if 7x - 9 we're solded. We remove 7x at 8x so at least we have only one x and we remove 9 than 16 so we have 7. That's why the reply is x + 7. Sorry if I wasn't clear, I am not really good at english.