(4x^2y+5xy-7x)-(5x^2y+4xy+x)
=4x^2y+5xy-7x-5x^2y-4xy-x
=-x^2y+xy-8x
;(6a^2-8a+11b^3)-(11a^2+2b^3)
=6a^2-8a+11b^3-11a^2-2b^3
=-5a^2-8a+9b^3