By Newton's second law, the net force acging upon a moving body is equal to the mass of the body times the acceleration of the body. Thus, F=ma. With a known force and acceleration, this is just a simple algebra step. I will use capital M for mass to prevent unit confusion.
F = M a
10N = M (2m/s^2)
5Ns^2/m = M
As a newton is defined as:
N = kgm/s^2
5Ns^2/m = 5kg
=> The mass of the body is 5 kilograms.
