X^2 + 4x - 12 = 0 is easily factorised into (x + 6)(x - 2). When we set this to 0 we can then say (x + 6) = 0 and (x - 2) = 0. At this point we can say for (x + 6) = 0 | (x) must equal x = -6. And for (x - 2) = 0 | (x) must equal x = 2. Put these in the place of (x). As an example if x = -6 then x^2 = 36. 4x = -24 this gives us the equation of 36 - 24 which equals 12. Taking the last integer of the equation 12 - 12 = 0.

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