a)
\(3x^2-3y^2\\ =3\left(x^2-y^2\right)\\ =3\left(x-y\right)\left(x+y\right)\)
b)
\(x^2-2xy+2x-4y\\ =x\left(x-2y\right)+2\left(x-2y\right)\\ =\left(x+2\right)\left(x-2y\right)\)
c)
\(4x^2-y^2+8y-16\\ =4x^2-\left(y^2-8y+16\right)\\ =4x^2-\left(y-4\right)^2\\ =\left(2x-y+4\right)\left(2x+y-4\right)\)