Chapter 9 - Quadratic Functions and Equations - 9-6 The Quadratic Formula and the Discriminant - Got It? - Page 583: 1

$x=7$ or $x=-3$

We are given: $x^2-4x=21$ $x^2-4x-21=0$ Use the quadratic formula. $x=\frac{-(-4) \pm \sqrt (-4)^2-4.1(-21)}{2.1}$ $x=\frac{4 \pm 10}{2}$ Simplify: $x=\frac{4 +10}{2}$ or $x=\frac{4 - 10}{2}$ $x=7$ or $x=-3$