
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
Given vectors $\overrightarrow{a}=(\sin x, \sqrt {3}\cos x)$, $\overrightarrow{b}=(3,-1)$.  
$(1)$ If $\overrightarrow{a}\parallel \overrightarrow{b}$, find the value of $\sin ^{2}x-6\cos ^{2}x$;  
$(2)$ If $f(x)= \overrightarrow{a}\cdot \overrightarrow{b}$, find the interval of decrease for the function $f(2x)$.

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