Using the binomial theorem, we know that the term will be of the form:
$\binom{18}{k}(3x)^k(-2y)^{18-k}$
We want to find the value of $k$ such that the term contains $x^5y^{13}$. This means that $k = 5$ and $18 - k = 13$. So, the term we are looking for
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