#### Answer

The domain of the rational function $$f(x)=\frac{P(x)}{Q(x)}$$ is
$$\mathcal{D}=\{x|x\in\mathbb{R},Q(x)\neq0\}.$$

#### Work Step by Step

Let $$f(x)=\frac{P(x)}{Q(x)}$$ where $P$ and $Q$ are polynomials. The rational function is defined at every real $x$ that satisfies $Q(x)\neq0$. In other words we have to throw away all the real zeros of the polynomial function $Q(x)$:
$$\mathcal{D}=\{x|x\in\mathbb{R},Q(x)\neq0\}=\mathbb{R}/\{x|Q(x)=0\}.$$