In any event, all you need to do is to assume that the person is feet along the ladder from the A. Then notice that the ladder forms a 3 - 4 - 5 triangle with the ground and the wall. So, by similar triangles, the man is feet horizontally from A and feet vertically.
Then, assume that the ladder is on the point of slipping, and resolve horizontally, resolve vertically, and take moments (about A, for instance). Solve for , and you're there.
Can you do it now?