since its x + 3 we use -3
-3| 2 1 -8 21 (these are the # without the x)
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-6 15 -21
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2 -5 7 0
Drag the 2 down, multiple it by -3 to get -6
1+(-6) = -5. -5 * -3 =15 . 15+(-8) = 7
7*-3 = -21. -21 +21 = 0
Since last # is 0 we have a factor.
factor is 2x^2 -5x -7 (we start with a power 1 less the original.)