1003097008

Find the first term of an arithmetic sequence ${\left\{a_n\right\}}_{n=1}^{\mathrm{\infty }}$, if the sum of the first eight terms is $12$ and the sum of the first twelve terms is $-6$: $$\begin{array}{rl}{s}_8& =12\\ s_{12}& =-6\end{array}$$