$\begin{array}{l}
\sqrt[3]{{m + \sqrt[3]{{m + 3\cos x}}}} = \cos x \Leftrightarrow m + \sqrt[3]{{m + 3\cos x}} = {\cos ^3}x\\
Dat\,\left\{ \begin{array}{l}
\sqrt[3]{{m + 3\cos x}} = u\\
\cos x = v
\end{array} \right. \Rightarrow m + 3v = {u^3}\\
\Rightarrow \left\{ \begin{array}{l}
m + u = {v^3}\\
m + 3v = {u^3}
\end{array} \right. \Rightarrow u - 3v = {v^3} - {u^3}
\end{array}$
=> Sai đề, bạn xem lại đề chỗ m+3cosx nhé!