Let $a,b\in R$ . If the mirror image of the point $P\left(a,6,9\right)$ with respect to the line $\frac{x-3}{7}=\frac{y-2}{5}=\frac{z-1}{-9}$ is , then  $\left|a+b\right|$is equal to 

$Midpoint\Rightarrow \left(\frac{a+20}{2},\frac{b+6}{2},\frac{-a}{2}\right)$is locate d on line ]

$\begin{array}{c}7\left(a-20\right)+5\left(6-b\right)+\left(-9\right)\left(18+a\right)=0\\ a+5b+272=0\end{array}$

$\begin{array}{c}\frac{a+20-6}{14}=\frac{b+6-4}{10}=\frac{-a+2}{18}\\ \left(a+14\right)18=14\left(a+2\right)\\ 4a+14\times 16=0\\ a=-56\\ b=-32\\ \left|a+b\right|=88\end{array}$