solve equation x^2-4x=21

we are working on polynomials and i think its called reverse factoring.
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1 Answer

x^2 - 4x = 21
x^2 - 4x - 21 = 0
x^2 - 7x + 3x - 21 = 0
(x^2 - 7x) + (3x - 21) = 0
x(x - 7) + 3(x - 7) = 0
(x + 3)(x - 7) = 0
x + 3 = 0 or x - 7 = 0
x = -3 or x = 7
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