\(a,x^2-y^2-4y-4\\ =x^2-\left(y^2+4y+4\right)\\ =x^2-\left(y+2\right)^2\\ =\left(x-y-2\right)\left(x+y+2\right)\\ b,x^2-y^2-6y-9\\ =x^2-\left(y^2+6y+9\right)\\ =x^2-\left(y+3\right)^2\\ =\left(x-y-3\right)\left(x+y+3\right)\\ c,4x^2-4y^2+12y-9\\ =\left(2x\right)^2-\left(2y-3\right)^2\\ =\left(2x-2y+3\right)\left(2x+2y-3\right)\)