\[\left( x^{- 2} - y^{- 2} \right)\ :\left( x^{- 1} - y^{- 1} \right) =\]
\[= \left( \frac{1}{x^{2}} - \frac{1}{y^{2}} \right)\ :\left( \frac{1}{x} - \frac{1}{y} \right) =\]
\[= \frac{y^{2} - x^{2}}{x^{2}y^{2}}\ :\frac{y - x}{\text{xy}} =\]