= 13xy - 11x² + 10y² + 15x² - 10xy + 15y²
= 13xy - 11x² + 10y² + 15x² - 10xy + 15y²
= (13xy - 10xy) + (-11x² + 15x²) + (10y² + 15y²)
= 3xy + 4x² + 25y²