well, from the sequence, notice 70,63,56,49,…
is simply dropping on the next term by 7 units, so,
70 - 7, 63
63 - 7, 56
and so on
thus, the "common difference", or the number you "add" to get the next term is -7..... as from the sequence itself, you can see the first term's value is 70.
[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}\\
----------\\
a_1=70\\
d=-7\\
n=49
\end{cases}
\\\\\\
a_{49}=70+(49-1)(-7)\implies a_{49}=70-336\implies a_{49}=-266[/tex]
