Firstly your answer to B is wrong, the stiffness constant stays constant. What is happening is that someone has applied an extra force to displace the mass, and then removed that force so that it oscillates. Do you understand what is is you'v done wrong here?
For (c), do you know the equation x = Acos(wt)? therefore, using differentiation, v = -Awsin(wt)
This will be max when sin = 1
Vmax = Aw
Here, t is time, w is angular frequency (or 2*pi*frequency) = root(k/m), A is amplitude, x is displacement
therefore Vmax = Aw = 20mm * root(245.25/2.25)