a: \(=3x^2-3y^2=3\left(x-y\right)\left(x+y\right)\)
c: \(=\left(x^2-y^2\right)^2-10\left(x^2-y^2\right)+25-4\left(x^2y^2+4xy+4\right)\)
\(=\left(x^2-y^2-5\right)^2-4\left(xy+2\right)^2\)
\(=\left(x^2-y^2-5-2xy-4\right)\left(x^2-y^2-5+2xy+4\right)\)
\(=\left(x^2-y^2-2xy-9\right)\left(x^2+2xy-y^2-1\right)\)