\[\boxed{\text{937.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[\textbf{а)}\ (3m - a)(a + 3m) -\]
\[- (2a + m)(3a - m) =\]
\[= 9m^{2} - a^{2} -\]
\[- \left( 6a^{2} - 2am + 3ma - m^{2} \right) =\]
\[= 9m^{2} - a^{2} - 6a^{2} - am + m^{2} =\]
\[= 10m^{2} - am - 7a^{2}\]
\[\textbf{б)}\ (x - 4y)(x + 3y) +\]
\[+ (x - 3y)(3y + x) =\]
\[= x^{2} + 3xy - 4xy - 12y^{2} +\]
\[+ x^{2} - 9y^{2} = 2x^{2} - xy - 21y²\]