In triangle abc, a = 9, c = 5, and angle b = 120á. find the measure of angle a to the nearest degree. show your work.

B = sqrt(a^2 + c^2 - 2ac cos B) = sqrt(9^2 + 5^2 - 2 x 9 x 5 x cos 120°) = sqrt(81 + 25 - 90cos120°) = sqrt(106 + 45) = sqrt(151) = 12.29

b/sin B = a / sin A
12.29/sin 120° = 9/sin A
sin A = 9sin 120° / 12.29 = 0.6343
A = arcsin(0.6343) = 39°