That would depend on whether the rope is moving.

If we assume that the rope is not moving, then the resultant 
force on the rope is 0.  

Is this a trick question?

Maybe you're asking for the tension in the rope if the rope is
not moving.

In that case, the tension is the sum of the forces on each side,
so it's 80N+70N=150N on each side, so the tension in the rope is
twice that or 300N.

Maybe you want to know what forces the boys on the other side are
pulling at.  If so, then:

   x + 2x = 150N
       3x = 150N
        x = 50N

So one boy is pulling at 50N and the other twice that at 100N. 

It is not clear what you are asking.  Maybe it really is a trick
question and the resulting force is 0, since the forces in
opposite directions cancel each other out.

Edwin