Determine acceleration from position vs time graph

[Momentum09]

Hi,
I was given a position vs. time graph of a projectile. I estimated the instantaneous velocity at the first 1/2 height, climax, and the second 1/2 height. From these slopes I created a velocity vs. time graph and use the slope of this line for my acceleration, but I keep getting this problem wrong.

Thanks!

 

[Integral]

OK,

Now if you want some help you will need to ask a question.