
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
In the book "Nine Chapters on the Mathematical Art," there is a problem about sending mail at a relay station. In modern terms, it can be expressed as follows: To send a document to a city 900 miles away, it takes one day longer than the specified time if a slow horse is used. If a fast horse is used, it takes 3 days less than the specified time. It is known that the speed of the fast horse is twice that of the slow horse. Find the specified time. If the specified time is denoted as $x$ days, the equation can be written as ______.

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