Since all of the sides are 15, and there's a right angle, we can assume the triangles are both the same size. So we can halve 15 to get the length of y, which would be 7.5. And here we can use the Pythagorean's Theorem to solve for x, by changing it to c^2-b^2=a^2, and we can fill out 15^2 - 7.5^2 = a^2, or 225 - 56.25 = √173.75, which is about 13.18.
