
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 vectors $\overrightarrow {a}=(1,2)$ and $\overrightarrow {b}=(x,-2)$, and $\overrightarrow {a}$ is perpendicular to $(\overrightarrow {a}- \overrightarrow {b})$:

(I) Find the magnitude of $|3 \overrightarrow {a}+ \overrightarrow {b}|$;

(II) If vector $\overrightarrow {a}-λ \overrightarrow {b}$ is parallel to $2 \overrightarrow {a}+ \overrightarrow {b}$, find the value of $λ$.

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