$\mathrm{}$ Driving down the road at a speed of $30\mathrm{.}2$ m$/$s$,$ you suddenly notice a fallen tree blocking the road a distance of $90\mathrm{.}1$ m ahead of you. You step on the brake pedal and decelerate at a constant rate. What must the magnitude of your acceleration be so that you will come to a stop $9\mathrm{.}7$ m in front of the tree?