Please help calculus optimization problem?

You are making 2 sides of the triangle: 1 you could call x and the other you could call 100-x. The height could be x sin theta, if theta is the angle formed with x and the base, or (100-x) sin alpha, if alpha is the angle formed with 100-x and the base. The area could be (1/2) 40 (x sin theta) and you are trying to maximize that. Then i get stuck! sorry.