
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 $\overrightarrow {a}$ and $\overrightarrow {b}$ be two non-collinear vectors. It is known that $\overrightarrow {AB} = 2\overrightarrow {a} + k\overrightarrow {b}$, $\overrightarrow {CB} = \overrightarrow {a} + 3\overrightarrow {b}$ and $\overrightarrow {CD} = 2\overrightarrow {a} - \overrightarrow {b}$. If points A, B, and D are collinear, find the value of $k$.

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