\[\left( - x^{7} - y^{3} \right)\left( y^{3} - x^{7} \right) =\]
\[= - \left( y^{3} + x^{7} \right)\left( y^{3} - x^{7} \right) =\]
\[= - \left( y^{6} - x^{14} \right) = x^{14} - y^{6}\]