Question

im trying to take the derivative of this problem and plug in seven but im not getting the answer.

s=t3–2t2–6t t> or equal to 0

where s is measured in meters and t in seconds. When does the particle reach a velocity of 7 m/s?

Answer 1

ds/dt = 3t^2 - 4t - 6

so when ds/dt = 7
3t^2 - 4t - 6 = 7
3t^2 - 4t - 13 = 0


Solve using the quadratic equation, and use the positive answer.
(I got 2.85 sec)

Answer 2

if I am driving a car at 100km / hr and my reaction time is 0.382 seconds.
with many thanks Stephen