The force F₁ produces an unknown acceleration by acting on the unknown mass

The force F₁ produces an unknown acceleration by acting on the unknown mass. The force F₂ produces the same unknown acceleration, by acting on the second unknown mass. Find the force which produces the same acceleration, by acting on these masses coupled together as a whole.
a) Derive a formula for the desirable force in terms of known forces.
b) Calculate the solution if F₁=3N, F₂=6N.