a car is travelling in the $+$x direction with constant acceleration. At times t$1$ and t$2$ the car positions are x$1$ and x$2.$ Prove the acceleration of the car is a$=2($x$2$t$1-$x$1$t$2)/$t$1$t$2($t$2-$t$1).$Assume t$0=0,$ and x$0=0.$