\(\Leftrightarrow4\left(1-cos^22x\right)+12cos^22x-7=0\)
\(\Leftrightarrow8cos^22x-3=0\)
\(\Leftrightarrow4\left(cos4x+1\right)-3=0\)
\(\Leftrightarrow cos4x=-\frac{1}{4}\)
\(\Leftrightarrow x=\pm\frac{1}{4}arcos\left(-\frac{1}{4}\right)+\frac{k\pi}{2}\)