Let a' be the acceleration of the sphere's cm wrt the wedge. It will be directed down the plane. Let a be the acceleration of the wedge. Let be the angular acceleration  of  the sphere. Now as the sphere is observed in the reference frame of the wedge, there is a pseudo force ma on the sphere directed opposite to the acceleration of the wedge.

If  N be the normal force between the sphere and the wedge,

N=mg cos - ma sin and Nsin = ma

This gives a = gsincos/(1+sin²)

Now, for no slipping a' = R

a cos + g sin - (f/m) = (5f/2m)

Substitute the value of a and find f