Physics problem: tension.

[Fanekaz]

1. The problem statement, all variables and given/known data
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.


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

3. 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 cant get 12N.............
Sorry about my poor english i'm portuguese.
Please somebody help me.
1. The problem statement, all variables and given/known data



2. Relevant equations



3. The attempt at a solution

[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