Question: How to use dsolve and diff together

Hi, I have the following simple differential equation. 

2x dx -9y^2 dy = 0

How can I enter the command to solve it? I know I'm supposed to use dsolve command, but I keep getting an input error saying that it expected an ODE. Google says that said message is because for whatever reason Maple cannot understand dx or dy, and that instead I need to use diff command. But when I enter: 

2x diff(x) - 9 y^2 diff(y) = 0

I get another error. I have tried other combinations, but at times I get errors like y(x) and y cannot both appear in the given ODE, which I don't understand why they can't as they are like basic run of the mill ODEs, so I'm a bit confused. 

I have also checked Maple's docs but they don't help either, I tried the first example given here: 
https://www.maplesoft.com/support/help/Maple/view.aspx?path=dsolve
 and I got the same "expecting an ODE or a set or list of ODEs" as in my own examples, so I'm guessing the docs are assuming steps or some configuration. 

How am I supposed to enter the command? Thanks in advance!