y=ax^2+bx+c
=a[x^2+bx/a]+c
=a[x^2+bx/a+(b/2a)^2-(b/2a)^2]+c
=a[(x+b/(2a))^2-b^2/(4a^2)]+c
=a[x+b/(2a)]^2-b^2/(4a)+c