recall that the degree of a polynomial or a monomial in this cases, comes from the term with the highest degree.
now, this is a monomial, and is just one term, its degree will come from the sum of all its variables' exponents, so we can see this one as,
![\bf -9\implies -9x^0\textit{ or }-9y^0\impliedby \textit{the exponent of the variable is }\stackrel{\stackrel{degree}{\downarrow }}{0}](https://img.qammunity.org/2019/formulas/mathematics/middle-school/gpsqi825movrzb4bt9ze8v6wldnjei5g0i.png)
recall that anything raised to 0 is 1, except for 0 itself, so -9x⁰ is really -9*1 or -9.