As shown in the figure, on a smooth horizontal plane, two objects AB move under the action of constant horizontal forces F1 and F2. Given that F1 > F2, what is the force a exerts on B

As shown in the figure, on a smooth horizontal plane, two objects AB move under the action of constant horizontal forces F1 and F2. Given that F1 > F2, what is the force a exerts on B

I guess it's probably a connectome problem
Let the force between ab be f and the common acceleration be a
For the whole: F1-F2 = (MA + MB) * a
For B: f-f2 = MB * a
So f = (MBF1 + maf2) / (MA + MB)