Finding instantaneous velocity

[K_Swiss]

A rocket is fired from the top of a building so that its height "h", in meters, at time "t", in seconds is given by "h(t) = - t^2 + 6t + 7"

e) Using the derivative function find the instantaneous velocity when the rocket hits the ground.

Derivative Function: "m = - 2t + 6"

Textbook Answer: -8m/s

My work:
f(0) = -2(0) + 6
f(0) = 6m/s

What am I doing wrong? Please correct my answer.

 

[skeeter]

My work:
f(0) = -2(0) + 6
f(0) = 6m/s

What am I doing wrong? Please correct my answer.

the rocket doesn't hit the ground at t = 0

h = -t[sup:3u2ua2vk]2[/sup:3u2ua2vk] + 6t + 7

0 = -t[sup:3u2ua2vk]2[/sup:3u2ua2vk] + 6t + 7

0 = (7 - t)(1 + t) ... t = 7

h'(t) = v(t) = -2t + 6

h'(7) = v(7) = -14 + 6 = -8 m/s