c) x^4 – y^4 + x^3y – xy^3.

c) x⁴ – y⁴ + x³y – xy³

= (x⁴ + x³y) – (y⁴ + xy³)

= x³(x + y) – y³(x + y)

= (x + y)(x³ – y³)

= (x + y)(x – y)(x² + xy + y²).