A cart is driven by a large propeller fan, which can accelerate or decelerate the cart.$$ The cart starts out at the position x$=0$ m$,$ with an initial velocity of $+[$var:fanspeed$0]$ m$/$s and a constant acceleration due to the fan. The direction to the right is positive. The cart reaches a maximum position of x$=+[$var:fanposition$]$ m$,$ where it begins to travel in the negative direction.$$ Find the acceleration of the cart. Round your answer to three significant figures.