Question 161377
(9x^4 + 3xy - y^3) - (x^4 + 7xy + 3y^3)
Use the distributive property to multiply the - sign to all the terms following the -sign

-x^4-7xy-3y^3

Then combine like terms and add to simplify
(9-1)x^4+(3-7)xy+(-1-3)y^3

8x^4-4xy-4y^3