Is this equation solvable and if so, how?

xy'=3y+(cosx)x^4

I don't understand how to solve it since it's non-separable.

y'/x^3-3y/x^4=cosx

d/dx(y/x^3)=cosx

y/x^3=sinx

y=(sinx)x^3

I don't understand how to solve it since it's non-separable.

There's more to Differential Equations than just the separable ones. A LOT more.

Surely

d/dx(y/x^3)=cosx

y/x^3=sinx+C

y=(sinx+C)x^3

where C is an arbitrary constant.