Question 171345
(-5x^3 + 2y^3) - (15x^3 - 7x^2y + 8xy^2 - 4y^3)

I would change the sign of all the terms after the negative sign
- (15x^3 - 7x^2y + 8xy^2 - 4y^3)=(-15x^3+7x^2y-8xy^2+4y^3)


Then I would combine like signs.


(-5-15)x^3+7x^2y-8xy^2+(2+4)y^3=-20x^3+7x^2y-8xy^2+6y^3