You don't "plug" the time in to any formula. The problem tells you that "s(t)" is the distance the car has gone, from the first police car, in time, t, in hours. You are told that the car passes the second police car in 1/15 of an hour so that s(0)= 0 (0 hours after passing the first car- i.e when he is at the first police car, his distance from that car is 0 miles) and s(1/15)= 5 (1/15 hour after passing the first police car, he is at the second police car so has gone 5 miles).