\(\Leftrightarrow\sqrt{2}cos\left(3x-\dfrac{\pi}{4}\right)=\sqrt{2}cos2x\)
\(\Leftrightarrow cos\left(3x-\dfrac{\pi}{4}\right)=cos2x\)
\(\Leftrightarrow\left[{}\begin{matrix}3x-\dfrac{\pi}{4}=2x+k2\pi\\3x-\dfrac{\pi}{4}=-2x+k2\pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{4}+k2\pi\\x=\dfrac{\pi}{20}+\dfrac{k2\pi}{5}\end{matrix}\right.\)