`a) 4x - 12 = 4(x - 3) `
`b) 5(y-3) - x(3-y)`
`= 5(y-3) + x(y-3) `
`= (x+5)(y-3) `
`c) x(x+y)^2 - y(x + y)^2 + x^2 y - x^3`
`= (x-y)(x+y)^2 + (x^2 y - x^3)`
`= (x-y)(x+y)^2 + x^2 ( y - x)`
`= (x-y)(x^2 + 2xy + y^2) - x^2 (x-y)`
`= (x-y)(x^2 + 2xy + y^2 - x^2) `
`= (x-y)(2xy + y^2)`
`= y(x-y)(2x + y)`