Answer:
x² + 4x - 21 = 0
x² + 7x - 3x - 21 = 0
x ( x + 7 ) -3 ( x + 7 ) = 0
( x - 3 ) ( x + 7 ) = 0
∴ x - 3 = 0 or x + 7 = 0
x = 0 + 3 x = 0 - 7
x = 3 x = -7
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