the function is quadratic, hence the graph of y is a parabola that opens upward in this case. the y-value of the vertex of the parabola is the least value in the range.

y = (x + 1)(x - 2)

the vertex is located at x = 0.5, midway between the two roots of the function.

the y-value of the vertex is y = (0.5 + 1)(0.5 - 2) = (1.5)(-1.5) = -2.25