\(\begin{array}{l} - 2{x^3}{y^4}:D = x{y^2}\\ \Rightarrow D = - 2{x^3}{y^4}:x{y^2} = - 2{x^2}{y^2}\end{array}\)
\(\begin{array}{l}\left( {10{x^5}{y^2} - 6{x^3}{y^4} + 8{x^2}{y^5}} \right):\left( { - 2{x^2}{y^2}} \right)\\ = \left( {10{x^5}{y^2}} \right):\left( { - 2{x^2}{y^2}} \right) - \left( {6{x^3}{y^4}} \right):\left( { - 2{x^2}{y^2}} \right) + \left( {8{x^2}{y^5}} \right):\left( { - 2{x^2}{y^2}} \right)\\ = - 5{x^3} + 3x{y^2} - 4{y^3}\end{array}\)