Simplifying
6m3n + -9m2n2 + -15mn3

Reorder the terms:
-15mn3 + -9m2n2 + 6m3n

Factor out the Greatest Common Factor (GCF), '3mn'.
3mn(-5n2 + -3mn + 2m2)

Factor a trinomial.
3mn((-1n + -1m)(5n + -2m))

Final result:
3mn(-1n + -1m)(5n + -2m)