das is part of a proof of pythagorean theorem
for a right triangle (has one interior angle as 90 degrees)

a and b are the lengths of the legs and c=length of hypotnuse (side oposite right angle)
we see that the lengths of the legs are 9in and 12in
a=9 and b=12 (or a=12 and b=9, doesn't matter)
we want to find c, or x in this case
so





x=15in
answer is 15 inches