Calculus

Use the position function s(t)=-4.9t^2+ 150, which gives the height (in meters) of an object that has fallen from 150 meters. Teh velocity at time t=a seconds is given by: lim as t approaches a s(a)-s(t) divided by a-t a) find the velocity of the object when t=3 b) at what velocity will the object impact the ground? Please help.. My teacher didn't explain this very well at all.