Hi sakonpure6!

If the velocity is 6 at the beginning and 12 at the end, the average velocity can hardly be 12!?!

Your answer of 9 is correct based on the information you gave.

It does require constant acceleration, but you have that, since it's a straight line.

Nope. That's the way to do it.And, in general how do you find the acceleration from a position- time graph?

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?

If the acceleration is constant, you only need 2 instantaneous velocities though.