[tex]\bf -7~~,~~\stackrel{-7+4}{-3}~~,~~\stackrel{-3+4}{1}~~,~~\stackrel{1+4}{5}~~,.....[/tex]
so, it's really adding 4 to get the next term, namely, 4 is the common difference, and the first term is -7.
[tex]\bf n^{th}\textit{ term of an arithmetic sequence} \\\\ a_n=a_1+(n-1)d\qquad \begin{cases} n=n^{th}\ term\\ a_1=\textit{first term's value}\\ d=\textit{common difference}\\[-0.5em] \hrulefill\\ a_1=-7\\ d=4\\ n=13 \end{cases} \\\\\\ a_{13}=-7+(13-1)4\implies a_{13}=-7+48\implies a_{13}=41[/tex]
