Answer -7
Explanation:
0 = x^2 + 4x - 21
0 = x^2 - 3x + 7x - 21
0 = x(x-3) + 7(x-3)
0 = (x - 3)(x + 7)
Therefore x - 3 = 0 or x + 7 = 0
so x = 3 or x = -7
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