\(a,\left(x-3y\right).\left(2xy-5y^2+4x\right)\)
\(=\left(x.2xy-x.5y^2+x.4x\right)-\left(3y.2xy-3y.5y^2+3y.4x\right)\)
\(=2x^2-5xy^2+4x^2-6xy^2+15y^3-12xy\)
\(=\left(2x^2+4x^2\right)-\left(5xy^2+6xy^2\right)+15y^3-12xy\)
\(=6x^2-11xy^2+15y^3-12xy\)
\(b,2x.\left(x-5\right)+\left(x-2\right).\left(x+3\right)\)
\(=2x^2-10x+x^2+x-6\)
\(=3x^2-9x-6\)