\(B=\dfrac{7x-9}{6x+y}+\dfrac{7x+9}{8x-y}\)
\(\left\{{}\begin{matrix}\dfrac{7x-9}{6x+y}=\dfrac{\left(6x+y\right)+\left(x-y-9\right)}{6x+y}=1+0\\\dfrac{7x+9}{8x-y}=\dfrac{\left(8x-y\right)-\left(x-y-9\right)}{8x-y}=1-0\\\Rightarrow B=2\end{matrix}\right.\)