Essential GCSE physics 13.10 - Isaac Physics

The thrust generated by a rocket engine is equal to the mass of propellant burnt each second multiplied by the exhaust velocity of the gas. The Space Shuttle (with booster rockets and external tank) had a total mass of 2040000kg at launch. In this question we shall assume that the exhaust velocity of the gas was 3000m/s, and we will take g=10N/kg.

How much propellant would have to be burnt each second in order for the spacecraft to accelerate upwards from the launch pad at "3g" (i.e. 30m/s)

Is anybody around to help me with this? I understand F = ma, but I dont get what u do if the mass isn't constant and we haven't covered differentiation yet in maths - which is what google says to do!
It'll be the initial acceleration of the rocket the question is asking about, so you don't have to worry about changing mass.
Use F = ma, where F is the initial resultant force on the rocket (remember to consider the initial weight of the rocket!), and m is the initial mass of the rocket