Friction problem (mass on top of another mass)

[braindead101]

a block with mass m1=4.0kg is placed on top of a block with mass m2=12.0kg.
(a) if the acceleration of the lower block is a2=5.2m/s^2, calculate the minimum coefficient of static friction (u minimum, where u is mew), for the contact surface between 1 and 2, that will prevent block 1 from slipping with respect to block 2.
(b) the contact surfaces are polished and it is found that block 1 now slides on block 2. if the coefficient of kinetic friction uk=1/2u(minimum), u(minimum) found in a, what is the acceleration of block 1 with respect to the table (a1).
(c) what is the acceleration of block 1 with respect to the acceleration of block 2?
(d) what force must be applied to block 2 in (a) and (b) if the table is frictionless

im stuck on a, i don't know what to do with the acceleration, i thought about multiplying it with one of the masses, but i don't know which one

 

[dx]

Draw free body diagrams for both the blocks. What forces are acting on each of them?

 

[braindead101]

for (a) i found the Ff of m1 by:
Ff = ma
Ff = 4(5.2)
Ff = 20.8
mew (u) = Ff/Fn
u = 20.8/(4)(9.8)
u = 0.53

so for (b) the u k = 1/2 (0.53) = 0.265

am i right for (a)? and (b), I've no idea how to start