$$\frac23 cos (3x-50^\circ) = \frac1\sqrt3$$ $$cos (3x-50^\circ) = \frac\frac1\sqrt3\frac23$$
$\frac\frac1\sqrt3\frac23=\frac1\sqrt3 : \frac23=\frac32 \cdot \sqrt3=\frac3 \cdot \sqrt32 \cdot \sqrt3 \cdot \sqrt3=\frac\cancel3 \cdot \sqrt32 \cdot \cancel3=\frac\sqrt32$
$$cos (3x-50^\circ) = (…)