\(\begin{array}{l}
a)\,\,\left( {{x^7}{y^8}z - 3{x^6}{y^6} - {x^6}{y^7}} \right):\left( {2{x^6}{y^5}} \right)\\
= {x^6}{y^6}\left( {x{y^2}z - 3 - y} \right):\left( {2{x^6}{y^5}} \right)\\
= \frac{1}{2}y\left( {x{y^2}z - y - 3} \right).\\
b)\,\,\left( { - {x^4}{y^5}z + 2{x^4}{y^6} - \frac{1}{2}{x^5}{y^5}} \right):{\left( { - xy} \right)^4}\\
= {x^4}{y^5}\left( { - z + 2y - \frac{1}{2}x} \right):{\left( {xy} \right)^4}\\
= y\left( { - z + 2y - \frac{1}{2}} \right).\\
c)\,\,\left( {20{x^5}{y^3}z - 18{x^3}{y^5}{z^2} - 15{x^4}{y^5}{z^3}} \right):\left( { - 3{x^2}{y^3}z} \right)\\
= {x^3}{y^3}z\left( {20{x^2} - 18{y^2}z - 15x{y^2}{z^2}} \right):\left( { - 3{x^2}{y^3}z} \right)\\
= - \frac{1}{3}x\left( {20{x^2} - 18{y^2}z - 15x{y^2}{z^2}} \right).
\end{array}\)
