Hello CeliaYou don't mention the weight of the ladder. Are we told what that is?

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?

Grandad