x2 + 10x + 21 = (x + 3)(x + 7)
You need to find 2 numbers which sum to 10 and whose product of 21.
In this case 3 + 7 = 10 and 3 x 7 = 21.
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x2 + 10x + 21 = (x + 3)(x + 7)
(x2+10x+21)/(x+3) = (x+7)
Factorising, we obtain, x2 + 10x + 21 = (x + 7)(x + 3). Thus, x2 + 10x + 21 / (x + 3) = x + 7. That's usually easier than dividing it out!
x2+10x+21 = (x+3)(x+7) when factored.....
x2+10x-24 = (x-2)(x+12) when factored