\[a_{1} = 5;\ \]
\[a_{n + 1} = a_{n} - 3\]
\[d = - 3;\]
\[a_{6} = a_{1} + 5d = 5 + 5 \cdot ( - 3) =\]
\[= 5 - 15 = - 10.\]
\[Ответ:a_{6} = - 10.\]