Let y=(4x-2)(x-4)=2(2x-1)(x-4)=4x2-18x+8=4(x2-9x/2+2),
y=4(x2-9x/2+81/16-81/16+2)=4(x-9/4)2-49/4
y+49/4=4(x-9/4)2 is in vertex form with vertex=(9/4,-49/4).
When y=0, 4(x-9/4)2-49/4=[2(x-9/4)]2-(7/2)2. We know from the original factors that the x-intercepts are ½ and 4. (The y-intercept is 8.)