\[\boxed{\text{751\ (751).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ \left( x^{4} + 7x^{2}y^{2} - 5y^{4} \right)\left( - 0,2xy^{2} \right) =\]
\[= - 0,2x^{5}y^{2} - 1,4x^{3}y^{4} + xy^{6}\]
\[\textbf{в)}\ \left( \frac{1}{3}a^{5}b - ab + \frac{1}{7} \right)\left( - 21a^{2}b^{2} \right) =\]
\[= - 7a^{7}b^{3} + 21a^{3}b^{3} - 3a^{2}b^{2}\]
\[\textbf{г)}\ \left( 0,5x^{7}y^{12} - 6xy - 1 \right)\left( - \frac{1}{6}\text{xy} \right) =\]
\[= - \frac{1}{12}x^{8}y^{13} + x^{2}y^{2} + \frac{1}{6}\text{xy}\]