How Do You Calculate the Acceleration of a Mass in a Pulley System?

[quiteacacophony]

Homework Statement

A 1.0 kg mass (mA) and a 4.0 kg mass (mB) are attached to a lightweight cord that passes over a frictionless pulley. The hanging masses are free to move. Find the acceleration of the larger mass.

Homework Equations

Fnet = ma = FB-FA
Fhanging object=m x acceleration due to gravity

The Attempt at a Solution

I've gotten 7.35m/s/s about thirty times now but the homework is on webassign and I only have one last try to get it right. It's high school physics, a college prep class, so I expect someone can figure it out.

Other

The masses are on opposite sides of the pulleys.

 

[learningphysics]

It's not 7.35m/s^2... can you show your steps. Then we can point out where you're going wrong.