Find all numbers for which the rational expression is undefined z^2+6/z^2-7z-8

Now.. if a fraction has a denominator that's 0, the fraction is undefined... so hmm for this to happen to this fraction, then it'd look like  

now, for that to happen, well, the denominator must be equal to 0, and that happens when z²-7z-8 = 0... when is that? well, let's check.