\[\frac{1^{\backslash 3x - y}}{3x + y} - \frac{1^{\backslash 3x + y}}{3x - y} =\]
\[= \frac{3x - y - 3x - y}{(3x + y)(3x - y)} =\]
\[= - \frac{2y}{9x^{2} - y^{2}} = \frac{2y}{y^{2} - 9x^{2}}.\]