the main formula for a polynomial ax^my^n+...
the degree is the sum of the highest degree of the polynomial, it is m+n
the degree of x^3y^2+7x^2y^5-3xy^8 is 9, because 3xy^8 is the term where we can find a highest value of power (8)
so the degree is degree of x, it is 1 + degree of y (it is 8)