Chapter 5 - Exponential and Logarithmic Functions - 5.2 One-to-One Functions; Inverse Functions - 5.2 Assess Your Understanding - Page 265: 3

All real numbers apart from $3$ and $-6$.

We know that the denominator cannot be $0$, hence we want $x^2+3x-18\ne0$. After factorizing the trinomial: $(x+6)(x-3)\ne0$. For this we need that $x+6\ne0$, $x\ne-6$ and $x-3\ne0$, $x\ne3$. Therefore the domain is the real numbers apart from $3$ and $-6$.