Chapter 3 - Quadratic Functions - 3.2 The Vertex of a Parabola - Exercises and Problems for Section 3.2 - Skill Refresher - Page 127: S6

$$n=6,\:n=-3$$

Completing the square, we find: $$n^2-3n-18+\left(-\frac{3}{2}\right)^2-\left(-\frac{3}{2}\right)^2 \\ \left(n-\frac{3}{2}\right)^2-\frac{81}{4}$$ Setting this equal to zero: $$\left(n-\frac{3}{2}\right)^2=\frac{81}{4} \\ n=6,\:n=-3$$