Answer: Hello mate!
you know that the equation is (or at least i think this is):
p(t) = 800 + 9t/90 + t^2
You want to know the "rate of change" after 6 hours.
We know that the rate of change is the derivative of P(t) with respect to t; this is
now we want the rate of change when t = 6, then we replace t by 6 in the derivate equation:
so the rate of change after 6 hours is 12.9
where
If the equation is wrong ( because you write P(t) = 800 1 + 9t 90 + t2, and i don really know how to iterprete it) tellme and we can do the derivate again :D