Answer:
x = 100
Step-by-step explanation:
The ratios of long side to hypotenuse are the same for all these triangles.
x/(x+10) = (x+10)/(x+21)
x^2 +21x = x^2 +20x +100 . . . . . multiply by (x+10)(x+21), expand
x = 100 . . . . . . . . . . . . . . . . subtract x^2 +20x
