\(a,\)
\(x^3y-y\)
\(=y\left(x^3-1\right)\)
\(=y\left[\left(x-1\right)\left(x^2+x+1\right)\right]\)
\(=y\left(x-1\right)\left(x^2+x+1\right)\)
\(b,\)
\(x^3y+y\)
\(=y\left(x^3+1\right)\)
\(=y\left[\left(x+1\right)\left(x^2-x+1\right)\right]\)
\(=y\left(x+1\right)\left(x^2-x+1\right)\)
\(c,\)
\(\left(x-y\right)^2-x\left(y-x\right)\)
\(=\left(x-y\right)^2+x\left(x-y\right)\)
\(=\left(x-y\right)\left(x-y+x\right)\)
\(=\left(x-y\right)\left(2x-y\right)\)