\[(88 - x) - 12 = 39\]
\[Подставим\ x = 50;8;37:\]
\[x = 50 \Rightarrow (88 - 50) - 12 =\]
\[= 38 - 12 = 26 \neq 39\]
\[x = 8\ \Rightarrow (88 - 8) - 12 =\]
\[= 80 - 12 = 68 \neq 39\]
\[x = 37\ \Rightarrow (88 - 37) - 12 =\]
\[= 51 - 12 = 39 - подходит.\]
\[Ответ:x = 37.\]