You have to trust the picture, specifically that the part of the graph shown is concave upward; that is, the derivative is increasing for negative x. Therefore, what is the greatest value of f'(x)? What is the smallest value of f'(x)?
You'll want to do the same with the rest of the graph as you made it for part (a). This will not change your answers as found in the given part of the graph; think about what that is.