let's take a peek of each term, to be specific, the exponent of the variables only, to see what degree each term is
[tex]\bf 13-4ab+5a^3b\quad
\begin{cases}
13\impliedby \textit{no variable, degree is 0}\\
4ab\to 4a^1b^1\impliedby \textit{1+1, degree is 2}\\
5a^3b\to 5a^3b^1\impliedby\textit{3+1, degree is 4}
\end{cases}[/tex]
so...which term has the highest degree? well, that one, is the degree of the polynomial
