180 inches. You would first determine the two missing sides. The short missing side can be found by subtracting 18 in. from 40 in. to get 22 inches. The longer missing side can be found by subtracting 10 inches from 50 inches to get 40 inches. Finally, since you are finding the perimeter, you would add each side (50+40+10+22+40+18 = 180 inches).