Help! logarithmic exponential function problems?

i really dont understand how to solve this problem. Your car is standing still on a straight, level strech of a highway. At time t=0s your floorboard the gas pedal. After 5 seconds, you are going to 40 kph. Let D be the difference between your car's speed and its top speed of 160 kph ume that d decreases exponentially with t. A) write a particualr equation expressing D in terms of t. b) how fast will you be going 18 seconds after you floor board the gas pedal.