Work Step by Step
$p^2-4p=21$ $p^2-4p-21=21-21$ $p^2-4p-21 = 0$ $(p-7)(p+3)=0$ $p-7=0$ $p-7+7=0+7$ $p = 7$ $p+3=0$ $p+3-3 = 0-3$ $p = -3$
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