$F(x)=3 \cdot \cos 2=\frac{\pi}{3}$

Step 1 ：\(F(x) = 3 \cdot \cos 2x = \frac{\pi}{3}\)

Step 2 ：\(\cos 2x = \frac{\pi}{9}\)

Step 3 ：\(2x = \arccos \left(\frac{\pi}{9}\right)\)

Step 4 ：\(x = \frac{1}{2} \arccos \left(\frac{\pi}{9}\right)\)

Step 5 ：\(x \approx 0.607\)

Step 6 ：\(\boxed{0.607}\)