Answer: x = 12
Explain:
Use Cosine Law. Think of it as big triangle.
a^2 = b^2 + c^2 - 2bcCosA
12^2 = 10^2 + 18^2 - 2(10)(18)CosA
144 = 424 - 360CosA
360CosA = 424 - 144
360CosA = 280/360
/ 360
Cos A = 0.777777777777778
A = cos^-1(0.777777777777778)
Angle A is about 38.9 degrees
Use SOH CAH TOA. Right angle triangle that contains the 38.9 degree angle.
TOA
Tan 38.9 = 10/x
x = 10/ tan 38.9
x = 12.4
Or rounded to 12.