
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
(Optional 4-5: Inequality Lecture) Given the function $f(x)=|x+3|+|x-a| (a > 0)$.
(I) When $a=4$, it is known that $f(x)=7$, find the range of $x$;
(II) If the solution set of $f(x)\geqslant 6$ is ${x|x\leqslant -4 \text{ or } x\geqslant 2}$, find the value of $a$.

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