
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 $\{a_{n}\}$ is a geometric sequence, and the sum of its first $n$ terms is $S_{n}$, $S_{3}=21$, $S_{6}=189$.  
$(1)$ Find the general formula for $\{a_{n}\}$;  
$(2)$ If $b_n=(-1)^na_n$, find the sum of the first $n$ terms of $\{b_{n}\}$, denoted as $T_{n}$.

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