https://www.youtube.com/watch?v=mxdIf3IXmfs Exact differential equations may not be linear or homogenous such that you cannot simply separate the variables, after rewriting the differential equation in the form Mdx+Ndy=0, you will go on and test for exactness, if the test for exactness does not fail, set a function U(x,y)=integral(Mdx)+k(y), note that k is not a constant of integration, the function U(x,y) is the original function that yield the differential equation. Understanding total differential is the key to understanding exact differential equation.

https://www.youtube.com/watch?v=mxdIf3IXmfs