y=x²-4x+3=x²-4x+4-1=(x-2)²-1
h=2,k=-1
y1-y2=(x1²-4x1+3)-(x2²-4x2+3)=(x1²-x2²)-4(x1-x2)=(x1+x2-4)(x1-x2)
∵x1