Physics Problem: Tension Calculation for Two Bodies on a Frictionless Surface

[Fanekaz]

Homework Statement

There are two bodies, A and B, body A as 4,0 kg and B as 8,0 kg, the bodies are in a horizontal surface, with no friction. Both bodies are connected by a non-extensible string with unknown mass, which doesn't matter to the case.
The bodies are moving under de action of a force which has 18,0N of intensity.
So I have to discover the intensity of the tension, which I happen to know the answer that is 12N, I just can't get there.

Homework Equations

Resulting force=Weight+wheight+Fn+Fn+t+t+Friction+F
I think so...

The Attempt at a Solution

I really don't know how to solve this, I tried several times and searched for some formulas but i can't get 12N....
Sorry about my poor english I'm portuguese.
Please somebody help me.

 

[kuruman]

The two masses move together with the same acceleration under the influence of a single force of 18 N. Can you find the common acceleration of the masses? That will be a good first step.

 

[Fanekaz]

thanks i will try that