Question:
Find the domain of $\frac{x^2 + 10x + 21}{x^2 + 4x - 21}$.  (Express your answer using interval notation.)

Answer:
We cannot divide by 0 so we have to exclude from the domain the values of $x$ that make the denominator 0. First we factor the denominator into $(x-3)(x+7)$. Then we set it equal to 0 and solve for $x$. We find that $x$ cannot be 3 or -7, so $x \in \boxed{(-\infty, -7)\cup(-7, 3)\cup(3, \infty)}.$