Essentially, you would take the formula for the laws of sines and cosines: a^2=b^2+c^2-2bc(Cos(theta)). a is the unknown side you are solving for and theta is the opposite angle to a. Since the angle opposite to R is supplementary to the 50 degree angle, 180-50=130. b could be assigned to either of the given sides; the same goes for c (e.g. b=40 and c=38). Thus, the question should be straightforward in plugging in quantities and solving for the resultant force.