Is it 3? Bottom base: 4+9=13, right-angled triangle, so z=12, x=5. If we know one side is 5, and the other side is 4, y=3..
Let's look at this one closely......
Here's a pic :
We have altitude BC drawn to a right angle in triangle ABD
Note that triangle ABC is similar to triangle CBD
AB / BC = CB / BD which implies that
4 / BC = BC / 9 cross-multiply
36 = BC^2 take the square root of both sides
6 = BC = "y"
Thus, altitide BC in this situation serves as a geometric mean to the two bases of each of the smaller triangles formed