\[12;8;4;\ldots\]
\[a_{1} = 12;\ \ a_{2} = 8;\ \]
\[d = 8 - 12 = - 4;\]
\[a_{4} = a_{1} + 3d = 12 - 12 = 0;\]
\[a_{5} = a_{1} + 4d = 12 - 16 = - 4;\]
\[a_{6} = a_{1} + 5d = 12 - 20 = - 8.\]