\(A=\left(x+y-1\right)3-\left(x-y+1\right)3+6x^2\left(y-1\right)\)
\(A=3\left(x+y-1-x+y-1\right)+6x^2\left(y-1\right)\)
\(A=3\left(2y-2\right)+6x^2\left(y-1\right)\)
\(A=3.2\left(y-1\right)+6x^2\left(y-1\right)\)
\(A=6\left(y-1\right)+6x^2\left(y-1\right)\)
\(A=6\left(y-1\right)\left(1+x^2\right)\)