This is the zero product property. Either x3 = 0 or x^{2} +4x 21 = 0. So solve each of the two equations, factoring the second term.
x3 = 0
x = 3
x^{2} +4x 21 = 0
x^{2}  3x + 7x 21 = 0
x(x  3) + 7(x3) = 0
(x3)(x+7) = 0
Using the zero product property, x = 3 or x = 7
6/7/2014

Alex S.
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