\[При\ x = 23:\]
\[56 - (x + a) = 28\]
\[56 - (23 + a) = 28\]
\[23 + a = 56 - 28\]
\[23 + a = 28\]
\[a = 28 - 23\]
\[a = 5.\]
\[Ответ:нужно\ подставить\ a = 5.\ \]