Slipping Ladder (mechanics)

A uniform ladder AB of length 2a and weight W rests with end A on horizontal ground and the end B against a vertical wall. The coefficients of friction at A and B are 2/5 and 1/2 respectively. The ladder makes an angle x with the horizontal, where tan x = 5/4. If a man of weight W starts to climb the ladder, find his distance from A when the ladder begins to slip.

Say we call friction at A = f1, whilst the normal reaction at A = R, then 2/5R=f1
If we call friction at B f2, and the normal reaction at B = N, 1/2N=f2
For equilibrium, f2+R=W and f1=N

What do I do next? I do know that when the man steps on the ladder the normal reaction at A increases, so friction increases, but how do I show that the ladder slips and where the man has to be when it slips? It should involve moments, I know lol

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