Conservation of Momentum: Pulley System

[Watney]

Homework Statement

Block A (of mass MA) rests on a frictionless horizontal table. It is connected via a light string to block B (of mass MB) hanging over the edge of the table. The pulley itself, a solid disc, has non-negligible mass MC. The light string does not slip over the pulley.
What is the magnitude of the linear acceleration of the system?

Edit: Messed up with the title! This problem is in the chapter of Conservation of Momentum in my book but I actually didn't use it for this problem.

Homework Equations

Στ=Iα
α=a/r
Moment of Inertia of a disk = 1/2mr^2

The Attempt at a Solution

So here's my attempt but I'm not sure where I'm messing up.

 

[Vibhor]

Your work looks correct .

Why do you think you are messing up somewhere ?

 

[Watney]

Your work looks correct .

Why do you think you are messing up somewhere ?

When I enter my answer into my online homework answer checker, it says it's wrong. I'll have to ask my professor if maybe there's a mistake with his website cause sometimes he messes up!
Thanks though!

 

[TomHart]

I don't know where you're messing up. I didn't see anything wrong.

 

[TomHart]

sometimes he messes up

That is probably what happened. I went through the problem twice and got the same result both times.

Acknowledging @Vibhor who answered before me. I didn't see it until after I posted.