Pls help with physics question!!?

With this initial speed and deccel. the rollercoaster will take (15/7) seconds to come to a stop. During this time, which is 2.143, the rollercoaster is doing an AVERAGE speed of ( (15 + 0)/2 ) metres per second, and therefore takes ( 7.5 metres per second X 2.143 seconds) = 16m.