This is a linear differential equation. Probably the simplest way to do it is to find an integrating factor. Can you find a function, [itex]\mu(x)[/itex] such that
[tex]\frac{d\mu y}{dx}= \mu\frac{dy}{dx}+\frac{d\mu}{dx}y= \mu\frac{dy}{dx}+(3\mu /x^3)y[/tex]?