Answer
$c=-20$
Work Step by Step
$$\begin{align*}
-21+11&=\frac{1}{2}c-11+11&\text{Add 11 to each side.}\\
-10&=\frac{1}{2}c &\text{Simplify.}\\
2(-10)&=2 \cdot \frac{1}{2}c&\text{Multiply 2 to both sides.}\\
-20&=c &\text{Simplify.}
\end{align*}$$
Check the answer by substituting $-20$ to the variable in the given equation.
$$\begin{align*}
-21&=\frac{1}{2}c-11\\
-21&\stackrel{?}{=}\frac{1}{2}\cdot(-20)-11\\
-21&\stackrel{?}{=}-10-11\\
-21&\stackrel{\checkmark}{=}-21
\end{align*}$$
