Answer:
6
Step-by-step explanation:
All of the triangles are similar, so the ratio of hypotenuse to short side is the same for all.
... x/4 = 9/x . . . . that ratio for smallest and largest triangles
... x^2 = 36 . . . . multiply by 4x
... x = 6 . . . . . . . take the square root
