Question

can you help me factor the expression x^2+4x-21

Answer 1

To factor this expression we need to find two numbers which sum is 4 and the product is -21.

Now, 21 has only 2 factors (besides 1 and 21), 7 and 3, se we need to write them in a way such that they sum 4, so

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

we see that 7-3=4 and 7(-3)=-21, so the factorization is correct.