To simplify the expression, we first factor the numerator and denominator:

Numerator: (m-3n)x(m²-n²) = (m-3n)x(m+n)(m-n) = (m-3n)x(m+n)(m-n) = m²(m+n) - 3n(m+n)

Denominator: (m+n)x(3m-9n) = (m+n)x3(m-3n) = 3(m+n)(m-3n)

The expression becomes: (m²(m+n) - 3n(m+n)) / 3(m+n)(m-3n)

Now, we can simplify the terms further:

m²(m+n) = m³ + m²n
3n(m+n) = 3mn
3(m+n)(m-3n) = 3(m²-9n²) = 3m² - 27n²

So, the simplified expression is: (m³ + m²n - 3mn) / (3m² - 27n²)