To factor, we must first figure out what two numbers have a sum of -4 and a product of -12. The answer to this is -6 and 2; therefore, we know we must use these numbers in our two factors. This method is shown below:
x^2 - 4x - 12
(x - 6) (x + 2)
To check our work, we can multiply the two factors we have found together and make sure that they recreate our original expression. (x-6)(x+2) = x^2 - 4x - 12, so our answer is correct.
Hope this helps!