Đáp án:
$\begin{array}{l}
a)3x\left( {{x^2} - 5x + 7} \right)\\
= 3{x^3} - 15{x^2} + 21x\\
b) - 2xy\left( {2{x^3} + 5x - 1} \right)\\
= - 4{x^4}y - 10{x^2}y + 2xy\\
c)\left( {x + 4} \right)\left( { - {x^2} + 6x + 5} \right)\\
= - {x^3} + 6{x^2} + 5x - 4{x^2} + 24x + 20\\
= - {x^3} + 2{x^2} + 29x + 20\\
f)\left( { - 12{x^3}{y^4} + 6x{y^2} - 18xy} \right):6xy\\
= \left( { - 2{x^2}{y^3} + y - 3} \right).6xy:6xy\\
= - 2{x^2}{y^3} + y - 3\\
g)\left( {6{x^2}{y^5} - x{y^3} + 4{x^3}{y^2}} \right):2xy\\
= 3x{y^4} - \frac{1}{2}{y^2} + 2{x^2}y\\
h){\left( {3x - 1} \right)^2} - 7\left( {{x^2} + 2} \right)\\
= 9{x^2} - 6x + 1 - 7{x^2} - 14\\
= 2{x^2} - 6x - 13
\end{array}$
