You have to use Pythagorean's theorem to find the missing side measure. 41^2=40^2 + w^2. 1681 = 1600 + w^2. 1681-1600=w^2. 81=w^2 and w = 9. That's an odd triangle, but accurate! Now use the cos definition of cos x = 9/41, and cos x = .2195. Now use the inverse function on your calculator in degree mode to see that the angle x is 77.3 degrees.