Question 231087
factoring 
N(t) = -3t^3 + 23t^2 + 8t
= -t(3t^2 - 23t + 8)
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and 

V(s) = s^3 -13s^2 + 54s - 72
= (s-3)(s-4)(s-6)
Solved by graphical and min-max inspection.