Rates of change question

[magmagod]

Use an algebric strategy to verify that the point given for each function is either a maximum or a minimum.

x^3-3x ; (-1, 2)

so i tried finding the instantaneous rate of change before -1 and after -1 to see if its a maximum or a minimum, since if its a local max before -1 would be positive and after would be negative but i couldn't reach the solution in the back of the book. any help would be appreciated.

the formula i used was the difference quotient.