
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
Let $\overrightarrow {e_1}$ and $\overrightarrow {e_2}$ be two non-collinear vectors in a plane, and $\overrightarrow {a} = x \overrightarrow {e_1} - 3 \overrightarrow {e_2}$ ($x \in \mathbb{R}$), $\overrightarrow {b} = 2 \overrightarrow {e_1} + \overrightarrow {e_2}$. If $\overrightarrow {a}$ is parallel to $\overrightarrow {b}$, then the value of $x$ is __________.

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