\[\boxed{\text{561\ (561).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ 3m^{4}n^{2} = 3 \cdot \left( m^{2}n \right)^{2}\]
\[\textbf{б)}\ 12x^{6}{}^{4}z^{2} = 3 \cdot \left( 4x^{6}y^{4}z^{2} \right) =\]
\[= 3 \cdot \left( 2x^{3}y^{2}z \right)^{2}\]
\[\textbf{в)}\frac{3}{4}m^{8}n^{4} = 3 \cdot \left( \frac{1}{4}m^{8}n^{4} \right) =\]
\[= 3 \cdot \left( \frac{1}{2}m^{4}n^{2} \right)^{2}\ \ \]