Let \(a,b\in \mathbb{R}\). If the mirror image of the point \(P(a,6,9)\) with respect to the line \(\dfrac{x-3}{7}=\dfrac{y-2}{5}=\dfrac{z-1}{-9}\) is \(Q(20,b,-a-9)\), then \(|a+b|\) is equal to:
\(90\)
\(86\)
\(88\)
\(84\)