Newton's second law states that the force is equal to the rate of change of momentum; for a constant mass, force equals mass times acceleration. In mathematical terms this means that:

where m is the mass and a is the acceleration.
In this case the mass is 2.10 kg and the acceleration is 5.20 m/s²; plugging these values in Newton's second law we have that:

Therefore, the force needed is 10.92 N