M2 Equilibrium Question

I have a diagram that looks like this and I need to prove that the coefficient of friction is less than or equal to tan(alpha). So far I have taken moments about A to get the normal contact force R at P:
(3l)*R = 2lcos(alpha)W
R=(2/3)cos(alpha)W

When I try to get the coefficient of friction I said that Ff=Wsin(alpha) when resolving parallel to the plane but when equating that to the coefficient of friction I get u = 3/2tan(alpha) not just tan(alpha)

Can anyone help?