-3/7x²-6/7xy-3/7y²
4m(2-n)-6+3n
x³-4x²-x+4
-⅛ a²+¼ ab-⅛ b²
3a(b-4)-2b+8
x³+3x²-x-3

1)-3/7x^2-6/7xy-3/7y^2=-3y^2-6xy-3x^2=3y^2+6xy+3x^2=(y+x)^2
2)4m(2-n)-6+3n=4m(2-n)-3(2-n)=4m-3
3)x^3-4x^2-x+4=x^3-x-4x^2+4=x(x^2-1)-4(x^2-1)=x-4
4)-1/8a^2+1/4ab-1/8b^2=1/8a^2-1/4ab+1/8b^2=1/4a^2-1/2ab+1/4b^2=(1/2a-1/2b)^2
5)3a(b-4)-2b+8=3a(b-4)-2(b-4)=3a-2
6)x^3+3x^2-x-3=x^3-x+3x^2-3=x(x^2-1)+3(x^2-1)=x+3