if velocity is decreasing then how acceleration be positive as the velocity-time graph is negative?

 

Say  a ball bounces back from the wall(point A) to the point B as shown:

 

 

The velocity-time graph shows a line with a positive (upward) slope (meaning that there is a positive acceleration); the line is located in the negative region of the graph (corresponding to a negative velocity). The acceleration-time graph shows a horizontal line in the positive region of the graph (meaning a positive acceleration).