calc help
posted by Adam .
the position function of a particle is given by
s=t^3–4t^2–7t t>=0
where s is measured in meters and t in seconds. When does the particle reach a velocity of 10 m/s?

Take the derivative:
v=3t^2 8t 7
put v=10 and rearrange it to have a quadratic in standard format, solve the quadratic with the quadratic formula. 
i got 1.39 and 4.06 but i still got it wrong