Answer:
A. 4√3
Step-by-step explanation:
All of the right triangles in this geometry are similar, so the ratios of hypotenuse to short side are the same:
(8+4)/x = x/4
48 = x^2 . . . . . . . . . multiply by 4x
√48 = 4√3 = x . . . . take the square root
