\[\boxed{\text{738\ (738).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[A = 2x - 1;\ \ C = 5x;\ \ \]
\[B = 3x + 1:\]
\[A + B - C =\]
\[= (2x - 1) + (3x + 1) - 5x =\]
\[= 2x - 1 + 3x + 1 - 5x = 0\]
\[C - B - A =\]
\[= 5x - (3x + 1) - (2x - 1) =\]
\[= 5x - 3x - 1 - 2x + 1 = 0\]
\[Значит:\ \ \ \ \ \]
\[A + B - C = C - B - A \Longrightarrow ч.т.д.\]