well, Newton's second law is sigma(f) = ma so you calculate the sum of all forces acting on the rocket. Mass is constant as stated in the problem (even though it really wouldn't be if it was burning fuel).

forces on the rocket:
1000000 N straight up
gravity straight down
air resistance straight down

so add up all the forces, plug in the mass, and solve for the acceleration.

just remember in physics to add up all the forces and that velocity (thus acceleration) is directional.