How Do You Solve a Frictionless Pulley System Problem?

[Yosty22]

Homework Statement

A light rope is attached to a block with mass 4.00 kg that rests on a frictionless, horizontal surface. The horizontal rope passes over a frictionless, massless pulley, and a block with mass m is suspended from the other end. When the blocks are released, the tension in the rope is 10.0N.
a.What is the acceleration of either block?
c.Find the mass m of the hanging block.

Homework Equations

ƩF=ma

The Attempt at a Solution

My attempt at solving the problem is attached. Can someone tell me where I went wrong?
(It got cut off at the bottom, it just says that my answer of 0.813kg is wrong, but I don't know why. THE CORRECT ANSWER IS: m=1.37kg

 

[Yosty22]

Sorry, My attempt at the solution is now attached.

 

[haruspex]

You've been inconsistent in your choice of directions. Which way is the mass m accelerating?

 

[Mentalist]

Recheck your free body diagram. Where are the blocks accelerating? Specifically, on what axis as I am seeing two different axis here and that is not correct.

EDIT: Sorry, didn't see haruspex's post when I responded, I did the same problem.