summer college math...help

Sorry for the many questions..I was frustrated and up late. Still I have had no sleep.

Here is an example:Before 1997, the Library of Congress sold copies of its 3x5 catalog cards to other libraries. Due to libraries using computer catalogs, the Library of Congress stopped selling such cards in March 1997. The number of cards sold (in millions) for various years are listed in the table below.

Year Number of Cards Sold (in millions)

1971 74.47

1976 39.82

1981 15.64

1986 8.08

1991 2.36

1996 0.57

Let f(t) represent the number of cards in millions in the year that is t years since 1970.

a) Find an equation for f.

b) Use your model to estimate the number of cards sold in 1960.

c) What is the base b of your model f(t)=ab^t? What does it mean in terms of the situation?

Thanks