0) I suspect you mean 'c=" rather than "t=" on the second.
1) Have you considered substituting the two given values?
2) Perhaps you should try a 1st Derivative.
A car is stopped at a traffic light. It then travels along a straight road so that its distance from the light is given by x(t) = bt^2 - ct^3 where b=2.40 m/s^2 and c = 0.120 m/s^3; (a) calculate the average velocity of the car for the time interval t = 0 to t = 10.0 s (b) calculate the instantaneous velocity of the car at t= 0 to t = 10.0 s.