To find undefined points, you need to know when the bottom of the fraction is equal to 0, because you can't divide by 0! (The numerator is irrelevant.) In order to do this, you can factor the denominator of x^2 - 4x - 12. What factors of -12 add up to -4? That would be -6 and 2, so you can factor it out to (x-6)(x+2). Because the coefficient of x in both of these cases is 1, you can take the shortcut of just taking the opposite sign of the two numbers being added to x in these factors, giving you 6 and -2 for your undefined values.
Let me know if you need a more thorough explanation, as I sort of skipped through some wordy things there to give a more concise answer.
