This is in my college algebra book but I figure it's more of a geometry problem so I posted it here. I'm horrible with story problems and I can't even figure out really how to set this up. After I set it up I shouldn't have a problem solving it.

A television screen measures 54 inches diagonally, and it's aspect ratio is 4 to 3. Find the width and the height of the screen.

I know I have to apply Pythagorean theorem, but I don't see how. If a^2+b^2=c^2 then I have to apply the 4:3 ratio on the left side and 54 to the right, but I don't know how to properly setup that equality.

I'd appreciate any help as I'm struggling to catch up in this algebra 2 course. We just started and theres a gap in what's taught in the previous course and this one, yet the arrogant teacher feels like we should be pros at all this already. If I was a pro I wouldn't be paying for the class!