What exactly is the problem asking you to solve for?

I'm assuming the problem is asking you to solve for x?

The drawing shows inscribed similar triangles, so you can use x/6 = 15/x to solve for x since the side lengths of similar triangles are proportional.

If you need help with similarity, you may want to check out this topic: https://www.khanacademy.org/math/geometry/hs-geo-similarity

Hope this helps!