a) n(0) = 20 because n(0) = 20(2)^0 and anything to the power 0 is 1. The reason that any number to the zero power is 1 is because any number to the zero power is just the product of no numbers at all, which is the multiplicative identity, 1
Thus, n(0) = 20(1) = 20. In terms of the situation, if 0 days have passed since Monday, 20 people know about the rumor.
b) n(-2) = 20(2)^-2 = 20* 1/(2^2)
=20/(2^2) = 20/4 = 5.
In terms of the situation, two days before Monday, 5 people know the rumor.