I remember from my high school teacher that to find minimum value of x in quadratic equation (ax^2 + bx + c), min. value of x has a formula and it can be found by -b/2a. I think Greg does not mention this in his videos.

e.g. y = x^2 + 4x + 4 which is (x+2)^2

finding min value of x: (-4)/(2*1) = -2.

e.g. y = x^2 - 5x - 6 which is (x-6)(x+1)

finding min value of x: (-5)/(2*1) = -2.5

Good luck in GRE preparation!