If the displacement of a particle Is given by, `X = A+ Bt^2`. Find the velocity at t = 2s.

The displacement(S) of a body in a time t is given by S=at^2+bt .Find the dimensions of a and b.

If s=2t^(3)-6t^(2)+a t+5 is the distance travelled by a particle at time t and if the velocity is -3 when its acceleration is zero, then the value of a is