\[b_{1} = 3,5;\ \ S_{n} = 7;\]
\[S = \frac{b_{1}}{1 - q}\]
\[7 = \frac{3,5}{1 - q}\]
\[7 - 7q = 3,5\]
\[- 7q = - 3,5\]
\[q = \frac{3,5}{7} = 0,5.\]
\[Ответ:q = 0,5.\]