
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 $\triangle ABC$, $a, b, c$ are the sides opposite to angles $A, B, C$ respectively. $\cos B = \frac{3}{5}$ and $\overset{→}{AB} \cdot \overset{→}{BC} = -21$.

(1) Find the area of $\triangle ABC$.

(2) If $a = 7$, find angle $C$.

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