If the domain of the function \( f(x) = \log_{7} \left( 1 - \log_{4} (x^{2} - 9x + 18) \right) \) is \( (\alpha, \beta) \cup (\gamma, \delta) \), then \( \alpha + \beta + \gamma + \delta \) is equal to:
\( 17 \)
\( 16 \)
\( 15 \)
\( 18 \)