Simple Newtonian mechanics problem

[Aikon]

Homework Statement

It is the review problem on p.131 of Physics for scientists and engineers ( Serway, 4th edition).
It is the classical problem of 2 blocks in an incline(with angle $\theta$) with masses 2m and m (in order from bottom to the top of the incline) and 1 block suspended trough a pulley with mass M.
He asks to find M and give the answer as function of m, g, $\theta$.

Homework Equations

I got to 2 equations, they are:
$Mg =4ma+3mg\cos\theta$

and

$a=\frac{g(M-3m\cos\theta)}{M+3m}$

I couldn't get to something that wasn't a form of M²+... and it get very strange.
Some tip?

The Attempt at a Solution

It is above.

 

[gneill]

Presumably the blocks on the incline are connected to each other?

Is there some information in the problem statement regarding the type of motion expected? Otherwise, there doesn't appear to be any constraint on the the mass M.

 

[Aikon]

Presumably the blocks on the incline are connected to each other?

Is there some information in the problem statement regarding the type of motion expected? Otherwise, there doesn't appear to be any constraint on the the mass M.

Thank you,

After i read your reply i went back to check the exercise and i noticed that the sistem is in equilibrium !
I need to be a little more attentive to the details.
it is really just a "simple Newtonian mechanics problem".