6(x-y)-12(x-y)^2 (a^2-b^3)-5(b^3-a^2)-3k(a^2-b^3)

6(x-y)-12(x-y)^2
6(x-y)=6x-6y
(x-y)²=x²-2xy+y²
12(x²-2xy+y²)=12x²-24xy+12y²
6x-6y-(12x²-24xy+12y²)=6x-6y-12x²+24xy-12y²

2)(a^2-b^3)-5(b^3-a^2)-3k(a^2-b^3)
a^2-b^3-(5*b^3-5*a^2)=a^2-b^3-5*b^3+5*a^2
-b^3-5*b^3=-6*b^3
a^2+5*a^2=6*a^2
3*(a^2-b^3)=3*a^2-3*b^3
(3*a^2-3*b^3)*k=3*a^2*k-3*b^3*k
6*a^2-6*b^3-(3*a^2*k-3*b^3*k)=6*a^2-6*b^3-3*a^2*k+3*b^3*k