f(x) = (x - 6) (x +2)
The two roots will be found at x = -2 and x = 6
And, because of symmetry, the x value of the vertex will be the average of these root values = [ 6 + -2] / 2 = 4/2 = 2
And the y value of the vertex = (2-6) (2 + 2) = -4 * 4 = -16
So.....the vertex is (2, -16)