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To solve the equation by factoring:
p^2 - 4p = 21
p^2 - 4p - 21 = 0
Now we need to factor the quadratic equation:
(p - 7)(p + 3) = 0
Setting each factor to zero:
p - 7 = 0
p = 7
p + 3 = 0
p = -3
So the solutions are p = 7 and p = -3.