Physics

The position of a toy locomotive mobing on a straight track along the x-axis is given the equation x=t^3-6t^2 +9t where x is in metters and t is in seconds. The net force on the locomotive is equal to zero when t is eqaul to
a) zero
b)2 s
c)3 s
d)4s
e)5s

Would the answer be b)2 s? I found the double derivative of the equation and set x to 0 and solved for t.

f'= 3t^2 - 12t + 9
f"=6t - 12

yes, t=2

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