#### Answer

\[y'' = - \cos x\]

#### Work Step by Step

\[\begin{gathered}
y = \cos x \hfill \\
\hfill \\
differentiate\,\,to\,\,find\,\,y{\,^,} \hfill \\
\hfill \\
y' = - \sin x \hfill \\
\hfill \\
differentiate\,\,to\,\,find\,\,{y^,}^, \hfill \\
\hfill \\
y'' = \,{\left( { - \sin x} \right)^\prime } \hfill \\
\hfill \\
y'' = - \cos x \hfill \\
\end{gathered} \]