Very sorry, I've posted the question wrong. My problems remain though...
The actual question should have read:
The conserved quantity is:
Here's what I did with it though, maybe this will help...
If you ignore the 'c'.
Then you can do the following.
then multiply through by [tex] y' [\math] to get
then substitute (ignoring c) to get
Which you can integrate to get:
Which gives the result. Don't know how to do it with the c though.