$15)$Constant acceleration: A soccer ball is released from rest at the top of a grassy incline. After $8\mathrm{.}6$ seconds, the ball travels $87$ meters and $1\mathrm{.}0$ s after this, the ball reaches the bottom of the incline. $($a$)$ What was the magnitude of the ball's acceleration, assume it to be constant? $($b$)$ How long was the incline?