Remark
First of all, we had best correct the labeling. The adjacent is the side making up the unknown angle, and the opposite is the side not making up the given angle (in this case, x)
So you have mixed up the opposite and the adjacent. You need to switch them.
Step Two (Bottom Diagram)
Sin(x) = opposite / hypotenuse
Sin(x) = 12 / 18
Sin(x) = 2/3
Sin(x) = 0.6666666666
x = sin-1(0.66666666)
x = 41.18103
Problem One (Top One)
Sin(x) = opp/hypotenuse
Sin(x) = 4/5
Sin(x) = 0.8
x = sin-1(0.8)
x = 53.13 Answer <<<<<