Let \(\overline{ab}\) be the number

\(\overline{ba}=\overline{ab}+\overline{ba}-\overline{ab}=10a+b+10b+a-c\left(a+b\right)\)

\(=11\left(a+b\right)-c\left(a+b\right)=\left(11-c\right)\left(a+b\right)\)

So, the answer is 11 - c