Please HELP with this velocity question?

Question

The distance traveled by a car is given by d(t) = 40t^2 – 10t, where s is measured in kilometers and t in hours. After how many hours did the velocity of the car reach 70 km/h?

Answer 1

The velocity is the derivative of the distance with respect to time:

Let v be the velocity:

v=d'(t) = 80t - 10

Let's find t for which v=70 km/h

70 = 80t - 10 // + 10

80 = 80t // divide by 80

t=1 hour

Answer 2

d(t) = 40t^2 - 10t

d(t)/dt = 80t -10 = velocity

70 = 80t -10
80 = 80 t
t = 1 hour