Answer
$-5x^3+8xy-9y^2$ is of degree 3
Work Step by Step
$$(x^3+7xy-5y^2)-(6x^3-xy+4y^2)$$ $$=(x^3+7xy-5y^2)+(-6x^3+xy-4y^2)$$ $$=(x^3-6x^3)+(7xy+xy)+(-5y^2-4y^2)$$ $$=-5x^3+8xy-9y^2$$
Because the term with the largest degree is -5x^3, the degree of the expression is 3.
