To simplify the expression (6m^3 + 3mn^2)/(2m^3n + mn^3), we need to factor out common terms from the numerator and denominator.

First, let's factor out the numerator:

6m^3 + 3mn^2 = 3m(2m^2 + n^2)

Next, let's factor out the denominator:

2m^3n + mn^3 = mn(2m^2 + n^2)

Now, we can rewrite the expression as:

(3m(2m^2 + n^2))/(mn(2m^2 + n^2))

Since both the numerator and denominator have a common factor of (2m^2 + n^2), we can cancel them out:

= 3m/mn
= 3/n

Therefore, the simplified expression is 3/n.