Find the middle terms in the expansion `(3x-(x^3)/6)^7` .

Explanation

To find the middle terms in the expansion of \((3x - \frac{x^3}{6})^7\), we will use the Binomial Theorem. Let's go through the solution step by step.

### Step 1: Identify the terms in the binomial expansion
The Binomial Theorem states that for any expression of the form \((a + b)^n\), the expansion can be expressed as:
\[ \sum_{r=0}^{n} \binom{n}{r} a^{n-r} b^r \]
In our case, we can let:
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