I am starting to panic....

How do you find the average velocity from a Velocity-time graph. The graph has a slope of 0.75 and starts at (0,6) and ends at (8,12)

I calculated V.avg = 9. (( 6+ 12)*0.5) but the answer sheet says 12? How?

And, in general how do you find the acceleration from a position- time graph?

edit: I was looking at the wrong answer sheet >.< for the first part it is 12!

but I still don't know how to find the acceleration from a position - time graph, easily. I know I am supposed to find the instantaneous velocity at several points and then construct a velocity - time graph. But is there an easier way?