The factored equation becomes: (x + 3)(x - 4).
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First, find the 2 numbers that add to -1 and multiply to -12. The numbers are -4 and +3.
Split x^2 - x - 12 using these factors;
(x^2 + 3x)(-4x - 12)
Factor out x from terms in the first parentheses and factor out -4 from terms in the second set of parentheses.
x(x + 3) + -4(x + 3).
Take out the common term (x+3) and we are left with:
(x + 3)(x - 4).
x^2 - x - 12 factored = (x+3)(x-4)