`a, x^2 - xy + 3x - 3y`
`= ( x^2 - xy ) + ( 3x - 3y )`
`= x ( x - y ) + 3 ( x - y )`
`= ( x - y ) ( x + 3 )`
`b, x^2 + 4x - 21`
`= x^2 + 4x + 4 - 25`
`= ( x^2 + 4x + 4 ) - 25`
`= ( x^2 + 2 . x . 2 + 2^2 ) - 5^2`
`= ( x + 2 )^2 - 5^2`
`= ( x + 2 - 5 ) ( x + 2 + 5 )`
`c, x^3 - 4x^2 - xy^2 + 4x`
`= x ( x^2 - 4x - y^2 + 4 )`
`= x [ ( x^2 - 4x + 4 ) - y^2 ]`
`= x [ ( x^2 - 2 . x . 2 + 2^2 ) - y^2 ]`
`= x [ ( x - 2 )^2 - y^2 ]`
`= x ( x - 2 - y ) ( x - 2 + y )`
