What Is the Force of Tension Between Mass A and B?

[canadiankid]

Homework Statement

Force of tension externally is 150N [up]
Acceleration is 2.99 m/s^2 [down]
Mass A is the highest and is 4kg
Mass B is attached to Mass A by a light (negligible) string, Mass B is 6 kg
Mass C is attached to Mass B by a light (negligible) string, Mass C is 12 kg

What is the force of tension on the string between Mass A and B.

Homework Equations

There isn't really a formula for tension I don't think... Which is why I'm confused. I have gotten the wrong answer 3 times already. I am not sure how the relationship between the three bodies works in this system

 

[NascentOxygen]

Have you drawn a large, neat diagram?

You can analyze these types of equations quit methodically. Let's consider the lower mass. It has its own weight pulling down, and a retarding force (tension in that lower rope) pulling upwards. The result of this tug o' war is a force that causes that 12 kg body to accelerate downwards at 2.99m/sec2.

So, you can apply F=ma, to determine the only unknown in this bit of the puzzle, viz., T3 or whatever you want to name the tension in the lower rope.

Now, turn your attention to the middle weight ...

Shortcuts are best avoided until you know what is going on.