
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
(The full score of this question is 14 points)

Given the function $f(x) = -x^3 + 3x^2 + 9x + a$.

(I) Find the interval where $f(x)$ is monotonically decreasing;

(II) If the maximum value of $f(x)$ in the interval $[-2, 2]$ is 20, find its minimum value in this interval.

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