Chapter 11 - Rational Expressions and Functions - 11-2 Multiplying and Dividing Rational Expressions - Practice and Problem-Solving Exercises - Page 663: 63

$x=0, -4$

We are dividing the second fraction, so the numerator and denominator are flipped. Thus, there are two formulas that cannot equal zero in the denominator. Those formulas are $6x$ and $-3x-12$. If we set these equal to zero, then we find $x=0$ and $x=-4$, so these values cause the expression to be undefined.