To isolate the variable $a\text{,}$ we need to divide each side by $-2$ (because $a$ is being multiplied by $-2$). One common mistake is to add $2$ to each side. This would not isolate $a\text{,}$ but would instead leave us with the expression $-2a+2$ on the right-hand side. $$\begin{aligned} 10\amp=-2a\\ \divideunder{10}{-2}\amp=\divideunder{-2a}{-2}\\ -5\amp=a \end{aligned}$$ We will check the solution by substituting $a$ in the original equation with $-5\text{:}$ $$\begin{aligned} 10\amp=-2a\\ 10\amp\stackrel{?}{=}-2(-5)\\ 10\amp\stackrel{\checkmark}{=}10 \end{aligned}$$ The solution $-5$ is checked and the solution set is $\{-5\}\text{.}$