a: =28x^2+21xy-24xy-18y^2-2(14x^2-126xy+xy-9y^2)-247xy+19
=28x^2-250xy-18y^2+19-28x^2+250xy+18y^2=0
b: \(=6x-3-5x+15+18x-24-19x=-36\)
c: \(=3x^2+12x-7x+20+2x^3-3x^2-2x^3-5x\)
=20
d: \(=8x^2+12x-10x-15-4\left(2x^2+3x-2\right)+10x+7\)
=8x^2+12x-8-8x^2-12x+8
=0