A body attached to a spring creates a harmonic oscillator with a natural frequency that is dependent on the mass attached to the spring and the spring constant of the spring. If the mass of the body is m and it is attached to a spring with spring constant k the system has a natural frequency given by f = (1/2*pi)*sqrt(k/m)
Substituting the values given in the problem, the mass m that is attached to the spring is 20 g or 20/1000 = 0.02 kg and the spring constant is 12 N/m.
This gives the natural frequency as (1/2*pi)*sqrt(12/0.02)
=> (1/2*pi)*sqrt 600
The natural frequency of oscillations of the system created by a ball of mass 20 g attached to a spring with spring constant 12 N/m is 3.8984