y=x^2-4x+3=(x^2-4x+4)-1=(x-2)^2+1
y=-x^2+2x-3=(x^2+2x+1)-4=(x+1)^2-4
y=x^2-4x+c=(x^2-4x+4)+c-4=(x-2)^2+c-4
y=X^2+mx+4=(x^2+mx+m^2/4)+4-m^2/4=(x+m/2)^2+4-m^2/4