with the integral of (4x^2)/sqrt(-4x^2 + 16x -9)

why does the denominator factor into sqrt(7 - (2x-4)^2) I don't remember why that is. I know how to complete the square,

but i dont remember the process for a problem with a negative leading coefficient. I've worked it out several different ways and I still haven't gotten it to factor that way. Can anyone help? Please?