`a, x^2 + 4x - y^2 + 4`
`= (x+2)^2 - y^2`
`= (x+2+y)(x+2-y)`
`b, = x^2-4xy-4y^2+y`
`= (x-2y)(x+2y) - y(4x - 1)`.
`c, x^3-x+y^2-y`
`= x(x-1)(x+1) + y(y-1)`
`d, xz + yz-5(x+y)`
`= z(x+y) - 5(x+y)`
`= (z-5)(x+y)`
`e, x(x+3) - x - 3`
`= x(x+3) - (x+3)`
`= (x-1)(x+3)`