\[\boxed{\text{1027.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[\textbf{а)}\ 70a - 84b + 20ab - 24b^{2} =\]
\[= 14 \cdot (5a - 6b) +\]
\[+ 4b(5a - 6b) =\]
\[= (5a - 6b)(14 + 4b) =\]
\[= 2 \cdot (5a - 6b)(7 + 2b)\]
\[\textbf{б)}\ 21bc² - 6c - 3c^{3} + 42b =\]
\[= 3c^{2}(7b - c) + 6 \cdot (7b - c) =\]
\[= (7b - c)\left( 3c^{2} + 6 \right) =\]
\[= 3 \cdot (7b - c)(c^{2} + 2)\]
\[\textbf{в)}\ 12y - 9x^{2} + 36 - 3x^{2}y =\]
\[= 3y\left( 4 - x^{2} \right) + 9 \cdot \left( 4 - x^{2} \right) =\]
\[= (2 - x)(2 + x)(3y + 9) = 3 \cdot\]
\[\cdot (2 - x)(2 + x)(y + 3)\]
\[\textbf{г)}\ 30a³ - 18a^{2}b - 72b +\]
\[+ 120a = 6a^{2}(5a - 3b) + 24 \cdot\]
\[\cdot (5a - 3b) =\]
\[= (5a - 3b)\left( 6a^{2} + 24 \right) =\]
\[= 6 \cdot (5a - 3b)(a^{2} + 4)\ \]