Rút gọn biểu thức: a) (x+y) - (x -y)

a) \[\left( {x + y} \right)-\left( {x-y} \right) = x + y - x + y = 2y;\]

b) \[3x\left( {5{x^2} - 2x - 1} \right) - 15{x^3} = 15{x^3} - 6{x^2} - 3x - 15{x^3} = - 6{x^2} - 3x;\]

c) \[\left( {5x - 2y} \right)\left( {{x^2} - xy + 1} \right) + 7{x^2}y\]

\[ = 5x\left( {{x^2} - xy + 1} \right) - 2y\left( {{x^2} - xy + 1} \right) + 7{x^2}y\]

\[ = 5{x^3} - 5{x^2}y + 5x - 2{x^2}y + 2x{y^2} - 2y + 7{x^2}y\]

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