Can someone help me figure this out? (Geometry)

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

Thus

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