(a) Since the blocks are being moved at constant speed, then the forces are "balanced" making the friction force equal to the force P, so P = f = 6.9N.
(b) Kinetic friction is given as:
f = μn
Solved for μ:
μ = f / n
= f / 2mg------------------>treating the two blocks as one
= 6.9N / 2(2.5kg)(9.8m/s2)
(c) The frictional force acting on the upper block is static friction and must also equal the force P or it would slide to the left. The question doesn't ask for it, but the static friction coefficient must be:
μ ≤ f/mg
≤ 6.9N / (2.5kg)(9.8m/s2)
μ ≤ 0.28
Hope this helps.