Solving the coefficient of kinetic friction if tension in the string is known 

A block on an inclined surface is connected to another block that is
  hanging over the top edge of the incline, as shown in the following
  diagram. The blocks in the system are moving in such a way that block
  A (with a mass of 1.0kg) is moving upwards, as block B (with a mass of
  4.0 kg) slides down the ramp. If the rate of the acceleration is 1.2 m/s2, determine the magnitude of the force of tension in the string
  and the coefficient of kinetic friction.

So I was partially able to solve the following question.  I was able to determine the force of tension but I couldn't wrap my mind around solving the coefficient of friction.
 A: This seems like a "do my homework" question; here is the general method.
The coefficient of friction tells you the ratio between the normal force (this is the one perpendicular to the surface) and the frictional force (in the direction on opposite of motion). So, if you divide frictional force by normal force, you will have you answer.
You can find the frictional and normal forces by drawing a free-body diagram for the block on the incline. I recommend defining your x-axis such that it is colinear with the incline, as this will make the vector calculations easier.
Simply define you coodinates, list all the forces, set the sum of the forces equal to the acceleration, and solve for your two unknown forces. Once you finished with that you can find the ratio of frictional to normal forces and you will have your answer.
A: Draw Free Body diagrams for both. In this case both blocks are moving with the same acceleration of 1.2 m/s2. You will have 2 equations and 2 variables. Assume an arbitrary direction for the frictional force either upwards of downwards along the ramp. If the answer you get is negative then reverse the direction.
Anyway (assuming downward friction force)
Equation for Block B
$$mg \cdot \sin\theta+ f - T = ma \tag{1}$$
Equation for block A
$$T - Mg = Ma\tag{2}$$ 
$$f = uN = umg \cdot \cos\theta$$
Given $M = 10\, \mathrm{kg}$, $m = 4\, \mathrm{kg}$, $a = 1.2\, \mathrm{m/s^2}$
Solving 1 and 2 you will get the values of $f,$u and $T$.
A: So basically what I did here was draw FBDs for both blocks.  The first block gave me an equation and with the second block, I decided to find Fgb.y and Fbg.x (that's what the sine laws were for).  But I don't know how to go further than this.

