Sub in 5 for d and use a calculator to solve for N.
e is the base of the natural logarithm and all good calculators have a button for it (you can always use google if you don't have one to hand)
A rumor is spread at an elementary school with 1200 students according to the model
N = 1200(1 - e^-0.16d) where N is the number of students who have heard the rumor and d is the number of days that have elapsed since the rumor began. How many students will have heard the rumor after 5 days?