Simplifying
3mx2 + -5mx + -28m

Reorder the terms:
-28m + -5mx + 3mx2

Factor out the Greatest Common Factor (GCF), 'm'.
m(-28 + -5x + 3x2)

Factor a trinomial.
m((-7 + -3x)(4 + -1x))

Final result:
m(-7 + -3x)(4 + -1x)