7.27.
The initial potential energy of the block is mgh. The incline does work on the block in the form of friction. Conservation of energy gives us
Since the block starts and ends at rest, 
. The work
done by friction is given by
where we have used the fact that the normal force is given by
. Then the distance d is given by
pullling it all together we find that
Solving for 
yields the desired result.
