As an equation this is:
(F)(delta t) = (m)(delta v).
You are given the impulse
(F)(delta t) = 50 Ns
You are given that the bullet goes from 500 m/s to 0 m/s, so delta v is 500 m/s.
All you need to do is set up the equation:
50 Ns = 500(m) m/s.
Looks a lot like m = 0.1 kg
