#### Answer

$xy=-y^3+c$

#### Work Step by Step

Here, we have $x dy+y dx=-3y^2 dy$
The general solution can be determined when we multiply the equation $x dy+y dx=-3y^2 dy$ with the integrating factor and then integrate on the both sides.
we have $\int [xy]' =\int -3y^2 dy$
This implies that
$(xy)=-(y^3)+c$
Hence, $xy=-y^3+c$