Suppose f (x) = x³ + 2x² − x + 6. From calculus, the Mean Value Theorem guarantees that there is at least one number in the open interval ( −1, 2) at which the value of the derivative of f, given by f'(x) = 3x² + 4x − 1, is equal to the average rate of change of f on the interval. Find all such numbers x in the interval.