
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
In $\triangle ABC$, the sides opposite to angles $A$, $B$, and $C$ are denoted as $a$, $b$, and $c$ respectively, and it is given that $\cos \left(A- \frac{\pi}{3}\right)=2\cos A$.  
$(1)$ If $b=2$ and the area of $\triangle ABC$ is $3\sqrt{3}$, find $a$;  
$(2)$ If $\cos 2C=1- \frac{a^{2}}{6b^{2}}$, find the measure of angle $B$.

Present the answer in LaTex format: \boxed{Your answer}
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