Answer
$x = 3, -7$
Work Step by Step
$x^{2} + 4x = 21$
$x^{2} + 4x - 21 = 0$
Find two numbers that multiply to $-21$ and can be added together to $4$
$=-7$ and $3$
$x^{2} + 7x - 3x - 21 = 0$
$x(x+7) - 3(x+7) = 0$
$(x-3)(x+7) = 0$
$x = 3, -7$
Elementary Geometry for College Students (5th Edition)
by Alexander, Daniel C.; Koeberlein, Geralyn M.
Published by
Brooks Cole
ISBN 10:
1439047901
ISBN 13:
978-1-43904-790-3
Appendix A - A.4 - Quadratic Equations - Exercises - Page 560: 12
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