I have this exercise below that I need to find the x , y for.

$\displaystyle \frac{12}{X} = \frac{8}{Y} = \frac{4}{3}$

Now, I got to X + Y = 15

I did $\displaystyle \frac{12 + 8}{X + Y} = \frac{4}{3} $ And cross multiplication 

But from some reason I can't get to x or y...can someone please tell me why?

tnx