Show the graph of acceleration as a function of time T

Show the graph of acceleration as a function of time T

A block is pulled along a horizontal surface with a constant horizontal force of magnitude upper-case F. The surface exerts a frictional force of constant magnitude lower-case f on the block. The graph of speed v as a function of time t for the block is shown above. Write an equation that shows the graph of acceleration a as a function of time t for the block? 

Explain your answer.