Help with a business calculus problem

The percentage of households between 1980 and 2001 with VCRs can be modeled by the equation

P(t) = 84.430 / (1 + 33.605e^(-0.483921t)) percent

where t is the number of years since 1980.

a) Use the graph to estimate dP/dt when t = 7

b) Use the equation to investigate P'(7) numerically.

c) Interpret your answer to part b.

I wasn't in class to learn this lesson, I had a court date set... any help would be greatly appreciated...