Ans: A
$\begin{array}{cl}
& \dfrac{1}{3x + 7} – \dfrac{1}{3x – 7} \\
= & \dfrac{(3x – 7) – (3x + 7)}{(3x + 7)(3x – 7)} \\
= & \dfrac{3x – 7 – 3x – 7}{9x^2 – 49} \\
= & \dfrac{-14}{9x^2 – 49} \\
= & \dfrac{14}{49 – 9x^2}
\end{array}$
$\begin{array}{cl}
& \dfrac{1}{3x + 7} – \dfrac{1}{3x – 7} \\
= & \dfrac{(3x – 7) – (3x + 7)}{(3x + 7)(3x – 7)} \\
= & \dfrac{3x – 7 – 3x – 7}{9x^2 – 49} \\
= & \dfrac{-14}{9x^2 – 49} \\
= & \dfrac{14}{49 – 9x^2}
\end{array}$
