Newton's 2nd law of motion: Force = (mass) x (acceleration)
You need to find the acceleration, so divide each side by (mass):
Acceleration = (Force) / (mass)
Plug in the numbers you know from the question:
Acceleration = (300,000 Newtons) / (20,000 kilograms)
Acceleration = 15 m/s²
This is a huge acceleration. It's 53% faster than a falling rock (gravity).