\[a_{1} = - 12;\ \ a_{2} = - 6;\]
\[d = - 6 + 12 = 6;\]
\[a_{17} = a_{1} + 16d = - 12 + 16 \cdot 6 =\]
\[= - 12 + 96 = 84.\]
\[Ответ:a_{17} = 84.\]