The air exerts a forward force of 9 N on the propeller of a 0.40 kg model airplane. If the plane accelerates forward at 4.0 m/s2, what is the magnitude of the resistive force exerted by the air on the airplane?

I need help setting this up…please don't answer this with only the magnitude, please show me how you set it up.

Forward accelerating force on plane = m . a = 0.4 . 4 = 1.6 N

But propeller force on plane = 9 N

So resistive force must be 9 – 1.6 = 7.4 N

OK?