Calculus question that I don't quite understand

Question

The number, N, of people who have heard a rumor spread by mass media at time, t, is given by N(y) = a(1-e^kt).

There are 450,000 people in the population who hear the rumor eventually. 17 percent of them heard it on the first day. Find a and k, assuming t is measured in days.

Answer 1

If Daniel B.'s comment is correct:

then k is presumably negative so that a=450000

and ,17 = 1-e^k, i.e. k=ln .83.