It's a right triangle. Use the pythagorean theorem. 36^2+r^2=(r+12)^2. r=48.
X = 8.5 because 8.5 x 2 = 17 and 17 + 1 = 18

We effectively rewrite the equation as

In order for the LHS to be defined, we need to restrict
, or
. Now, the LHS will vanish when the numerator is 0, which happens for

This value is indeed smaller than 4, so the solution is
.
4,9,16 are all options that ratio to 1,2,3