Question 982865
X^2 + (x+6)^2 = (x+12)^2
FOIL (x+6)(x+6) and (x+12)(x+12)
x^2 + x^2 + 12x + 36 = x^2 + 24x + 144
Collect like terms on the left
x^2 + x^2 - x^2 + 12x - 24x + 36 - 144 = 0
x^2 - 12x - 108 = 0
You can use the quadratic formula a=1; b=-12, c=-108; but this will factor to
(x - 18)(x + 6) = 0 
It's the positive solution we want here
x = 18 is one side
then
18 + 6 = 24 is the other side
and
18 + 12 = 30 is the hypotenuse