In △ ABC, given a = 9, B = 2, C = 150 °, what is C equal to?

In △ ABC, given a = 9, B = 2, C = 150 °, what is C equal to?

According to the cosine theorem:
c²=a²+b²-2abcosC
=81+3-2×9×2×(-√3/2)
=84+18√3
∴c=√(84+18√3)