$$ \begin{aligned} -40 &= -5c-25&& \text{ swap the left and right side of the equation } \\[1 em]-5c-25 &= -40&& \text{ move -25 to the right } \\[1 em]-5c &= -40+25&& \\[1 em]-5c &= -15&& \text{ divide both sides by $ -5 $ } \\[1 em]c &= \frac{-15}{-5}&& \\[1 em]c &= 3&& \\[1 em] \end{aligned} $$