[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}\\
----------\\
d=5\\
a_3=8\\
n=3
\end{cases}
\\\\\\
a_3=a_1+(3-1)5\implies 8=a_1+(2)5\implies 8=a_1+10
\\\\\\
\boxed{-2=a_1}
\\\\\\
a_n=-2+(n-1)5[/tex]
