The triangle height is calculated by Pythagorean Theorem
height^2 = 15^2 - 9^^2
height^2 = 225 - 81
height^2 = 144
height = 12
We can calculate the value of Angle a
cosine (a) = adj / hyp = 9/15 = .6
arc cosine (a) = 53.13 degrees
angle b = 180 -90 -53.13
angle b = 36.87 degrees
tangent b = height / x
tangent 36.87 = 12 / x
x = 12 / .75
x = 16
Yes, it's just that "simple" LOL