Solve the following system of equations for $c$:
\begin{align*}
a - b &= 2 (c+d)\\
b &= a-2 \\
d &= c+5
\end{align*}
Substituting for $b$ and $d$ in the first equation gives $a - (a - 2) = 2(c+c+5)$. This simplifies to $2 = 4c + 10$, so $c = \boxed{-2}$.