We need to use the Law of Cosines:
cos (C) = (a^2 + b^2 -c^2) / 2 ab
cos (C) = (22^2 + 20^2 -18^2) / (2 * 22 * 20)
cos (C) = (484 + 400 - 324) / (880)
cos (C) = 560 / 880
cos (C) = 0.6363636364
arc cosine (C) = 50.479 Degrees
Source
1728.com/trigtut2.htm
(Scroll down to All 3 sides of a triangle)
