\input amstex
\def\akko{\;\Longleftrightarrow\;}
\def\pomod#1{\equiv_{#1}}
$$\align
12\pomod{7}5&\akko12=5+7\alpha,\\
8\pomod{7}1&\akko8=1+7\beta,\\
5+1&=6,\\
(5+7\alpha)+(1+7\beta)=6+7(\alpha+\beta)&\pomod{7}6,
\endalign$$
\bye