Dynamics question, test tommorrow

[Matt1234]

Dynamics question, test tomorrow :(

Hello, need some help on this one for the test. The solution is 3.9 m/s^2 in the book, I am not sure what i have done wrong.

Homework Statement

http://img196.imageshack.us/i/questionu.jpg/

Homework Equations

The Attempt at a Solution

Here is my attempt. sorry about the big X i was frustrated.

http://img269.imageshack.us/i/attempt.jpg/

 

[Matt1234]

hmmmmmmmmm, got it.

At the very end where i have Fnet = m a

m is actually (2.5 + 5.5)