Forgive me if I err along the line, but PH or someone will be along to correct me if I wonder.

It's been awhile.

Now, check to see if your DE is exact.

There exists a function f(x,y), such that:

From the 1st equation, after integrating:

Take the partial derivative with respect to y of the last expression and set the result equal to N(x,y):

and

Therefore,

So,

Check to make sure I didn't boo-boo.