How to factorize a quadratic expression with an extra constant?

Question

Pretty simple question that I never thought id get stuck on. How do I factor $x^2 - 6x + 8 - d$ into a form $(x - a)(x - b)$ where $d$ is just an unknown constant ? Been looking online but it seems all the tutorials / videos but have not found anything useful.

EDIT: Im silly for not remembering the formula $x = \dfrac{-b \pm \sqrt{b^2 - 4ac}}{2a} $

Answer 1

$x^2 + x(a + b) + ab = (x + a)(x + b)$

then,

$x^2 - 6x + 8 - c = (x + a)(x + b)$ , where

$ab = 8 - c$

$a + b = -6$