Answer:
(x+7)(x-3)
Explanation:
You can factor this trinomial into two binomials. 7 * -3 multiply together to form -21 and they add together to form 4.
x² + 4x -21 = x² + 7x - 3x -3*7
= x*(x + 7) -3(x + 7)
=(x + 7)*(x-3)
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