Answer
$0$
Work Step by Step
Divide both sides by $2$ to obtain:
$\cos^{-1}{x} = \dfrac{\pi}{2}$
Recall:
$y = \cos^{-1}{x} \hspace{15pt} \to \hspace{15pt} x = \cos{y}$
$\text{where } \hspace{15pt} -1 \leq x \leq 1 \hspace{15pt} \text{and} \hspace{15pt} 0 \leq y \leq \pi$
Thus, using the definition above gives:
$\cos^{--1}{x}=\dfrac{\pi}{2}
\longrightarrow x = \cos{\dfrac{\pi}{2}}$
$x =\cos{\dfrac{\pi}{2}} = \boxed{0}$
