Thu gọn biểu thức:

a) \( - 3{x^2}{y^5}z:\left( {15x{y^3}} \right)\)\( = \frac{{ - 1}}{5}x{y^2}z\).

b) \(3{x^2}\left( {2x + y} \right) - 2y\left( {4{x^2} - y} \right)\)

\( = 6{x^3} + 3{x^2}y - 8{x^2}y + 2{y^2}\)

\( = 6{x^3} - 5{x^2}y + 2{y^2}.\)

c) \(\left( {x + 3y} \right)\left( {x - 2y} \right) - \left( {{x^4}y - 6{x^2}{y^3}} \right):{x^2}y\)

\( = {x^2} - 2xy + 3xy - 6{y^2} - \left( {{x^2} - 6{y^2}} \right)\)

\( = {x^2} + xy - 6{y^2} - {x^2} + 6{y^2}\)

\( = xy.\)