a, \(4x^3-5x^2y-xy=x\left(4x^2-5xy-y\right)\)
b, \(x^3y^4\left(x^2-y^3\right)-x^3y^3\left(x^4-y^4\right)\)
\(=x^3y^3\left[y\left(x^2-y^3\right)-x^4+y^4\right]\)
\(=x^3y^3\left(x^2y-x^4\right)=x^5y^3\left(y-x^2\right)\)
\(=x^5y^4-x^3y^7-x^7y^3+x^3y^7^{ }\)
=x5y4-x7y3
\(x^3y^4\left(x^2-y^3\right)-x^3y^3\left(x^4-y^4\right)\)
\(=x^5y^4-x^3y^7-x^7y^3+x^3y^7\)
\(=x^5y^4-x^7y^3\)