1003097007

Find the common difference of an arithmetic sequence ${\left\{a_n\right\}}_{n=1}^{\mathrm{\infty }}$, if the sum of the first seven terms is $42$ and $a_{10}=-4a_5$: $$\begin{array}{rl}{s}_7& =42\\ a_{10}& =-4a_5\end{array}$$