For this equation, we have one shorter side angle and the hypotenuse.
Do a^2 + b^2 = c^2
We would find the shorter side of one triangle first, and then add two of the product and that would be the value of x,
12^2 + b^2 = 15^2
144 + b^2 = 225
b^2 = 81
b= 9
This is half the x since we could only find one of the two triangle's shorter sides
9 + 9 = 18
X = 18 is your answer