\[- 3;\ - 1;1;\ldots\]
\[a_{1} = - 3;\ \ \ a_{2} = - 1;\ \]
\[d = a_{2} - a_{1} = - 1 + 3 = 2;\]
\[a_{n} = a_{1} + d(n - 1)\]
\[- 3 + 2(n - 1) = 15\]
\[2n - 2 = 18\]
\[2n = 20\]
\[n = 10.\]
\[Ответ:n = 10.\]