st20t243t3 for 0 is less than or equal to t is less than or

s(t)=20t^2-(4/3)t^3 (for 0 is less than or equal to t is less than or equal to 18)
What is the velocity of the train after 13 hours

V(t)=S\'(t) V(t)=40t-4t^2
I get -156 miles/hour

Why am I getting a negative I shouldnt right?

yes it is corect ==> the system is dececlarating

$s(t)=20t^2-(4/3)t^3 (for 0 is less than or equal to t is less than or equal to 18) What is the velocity of the train after 13 hours V(t)=S\'(t) V(t)=40t-4t^2 I$