#### Answer

$−5 m^3n+5mn$

#### Work Step by Step

Combining Like Terms
This problem deals with combining like terms. Like terms are terms that have the same variable raised to the same power. Note, by this definition, $3xy^2$ and $9xy^2$ are like terms, for these two terms have the same two variables raised to the same powers.
Now, back to the problem. Recall, if terms are like terms, we can add or subtract these terms just like we add or subtract numbers. Thus, because -$9m^3n$ and $4m^3n$ are like terms, we can add them to simplify the expression to $−5m^3n+5mn.$