If the velocity time graph has the shape AMB, what would be the shape of the corresponding acceleration-time graph?

Slope of v-t graph gives acceleration (Refer Fig. 1) 

For  AM, slope is constant and negative. Therefore, acceleration is constant and negative.

Similarly, for MB, slope is constant and positive. Therefore, acceleration is constant and positive.
⇒ Acceleration will be (Refer Fig. 2)