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