How many seconds after the driver applies the brakes does the car come to a stop?

The driver of a car traveling at 42 ft/s suddenly applies the breaks. The position of the car is s = 42t - 3t^2, t seconds after the driver applies the breaks. How many seconds after the driver applies the brakes does the car come to a stop?



A. 294 sec

B. 147 sec

C. 7 sec

D. 588 sec



Im not only trying to get the answer, which would be nice, but a brief explanation of how to knock this out =]How many seconds after the driver applies the brakes does the car come to a stop?If s = 42t - 3t^2 gives the postion of the car after a certain amount of seconds, we can use that to calculate where the car is at any given time



After 1 sec s = 42(1) - 3(1^2) = 39

After 2 sec s = 42(2) - 3(2^2) = 72

After 3 sec s = 42(3) - 3(3^2) = 99

After 6 sec s = 42(6) - 3(6^2) = 144

After 7 sec s = 42(7) - 3(7^2) = 147

After 8 sec s = 42(8) - 3(8^2) = 144



Which means that after 7 seconds have passed the car has come to a stop (assuming it's not moving backward as the formula technically suggests)