$8)($Extra Credit$)$ if a is ball thrown straight up on Mars at an initial velocity of $15$m $/$ s we can find its height in meters after x seconds with the function h$($x$)=15$x $-1\mathrm{.}8$x $^2$

a$.(4$pts$)$ Find the average velocity of the ball between x $=5$ and x $=7$ seconds

b$.(2$pts$)$ Explain what this tells us about the ball.

direction of the