[tex]\bf \textit{sum of an arithmetic sequence}
\\\\
S_n=\cfrac{n(a_1+a_n)}{2}\qquad
\begin{cases}
n=n^{th}\ term\\
a_1=\textit{first term's value}\\
----------\\
S_n=795\\
a_1=102\\
a_n=57
\end{cases}
\\\\\\
795=\cfrac{n(102+57)}{2}\implies 1590=159n
\\\\\\
\cfrac{1590}{159}=n\implies 10=n\\\\
-------------------------------[/tex]
so the nth term is really the 10th term, and we know that's 57, thus
[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}\\
----------\\
n=10\\
a_{10}=57\\
a_1=102
\end{cases}
\\\\\\
57=102+(10-1)d\implies 57=102+9d\implies -45=9d
\\\\\\
\cfrac{-45}{9}=d\implies -5=d[/tex]
so, that's the common difference... .so you'd surely know what the 3rd term is, notice the first one is 102.
