1) 7х(4у-х)+4х(х-7у)=28xy-7x^2+4x^2-28xy=-3x^2=-3x^2+0x+0
2) 4а(7х-1)-7(4ах+1)=28ax-4a-28ax-7=-4a-7
3) а( а+в)+в(а-в)=a^2+ab+ab-b^2=a^2+2ab-b^2
4) 5а(6а+3в)-6а(5в-2а)=30a^2+15ab-30ab+12a^2=42a^2-15ab
5) 8м(м+н)-3н(2м-4н)=8m^2+8mn-6mn+12n^2=8m^2+2mn+12n^2