Answer:
x = 3
Explanation:
The right triangles are all similar, so the ratio of short leg to hypotenuse is the same for both marked triangles.
short leg / hypotenuse = x/(x+3) = (x+3)/12
12x = (x +3)^2 . . . . . multiply by 12(x+3)
x^2 -6x +9 = 0 . . . . put in standard form
(x -3)^2 = 0 . . . . . . . factor
x = 3 . . . . . the value of x that makes the factor(s) zero