Not your average derivatives application question *trick question?

Question

A position function of a particle is given by
s=t^3 - 6t^2 + 9t +1, t is greater than or equal to 0
Find the velocity after 4 s.

I tried taking the derivative of s and got 3t^2 - 12t + 9 however when I substituted t for 4 and got 33 m/s, the answer key said it was 9 m/s. Whats going on? What makes this question different from your "average" calculus velocity question?

Answer 1

3*(4)^2-12(4)+9=
48-48+9=
9 m/s
I believe your substitution is incorrect.

Answer 2

Check your math when you substituted t = 4 in your velocity equation.

3(16) - 12 (4) + 9 = 9 , not 33