To factor the quadratic expression \( n^2 - 9n + 18 \), we are looking for two numbers that multiply to the constant term (18) and add up to the coefficient of the linear term (-9).
The factors of 18 that add up to -9 are -6 and -3.
So, we can write the expression as:
\[ n^2 - 9n + 18 = (n - 6)(n - 3) \]
Thus, the factored form of \( n^2 - 9n + 18 \) is:
\[ (n - 6)(n - 3) \]