I took your working and looked for integer answers.
\(\frac{1}{-35-2c} \begin{pmatrix} -7 & -2 \\ -c & 5 \end{pmatrix}\\\)
each of these elements must be an integer. so
\(\frac{-2}{-35-2c}\) must be an integer, I only looked at -2 because the absolute value of it is the smallest one.
so \( -2\le -35-2c \le 2\\ 33\le -2c \le 37\\ 16.5\le -c \le 18.5\\ -16.5\ge c \ge -18.5\\ -18.5\le c \le -16.5\\ \)
but c is an integer, so c might be -18 or -17
I checked both of these and they both worked.