
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 respectively $a$, $b$, and $c$. It is known that $\overrightarrow{a}=(\cos A,\cos B)$, $\overrightarrow{b}=(a,2c-b)$, and $\overrightarrow{a} \parallel \overrightarrow{b}$.
(Ⅰ) Find the magnitude of angle $A$;
(Ⅱ) If $b=3$ and the area of $\triangle ABC$, $S_{\triangle ABC}=3 \sqrt {3}$, find the value of $a$.

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