Chapter 1 - First-Order Differential Equations - 1.9 Exact Differential Equations - Problems - Page 92: 19

See answer below

If $phi(x, y)$ is a potential function for $$M(x,y)dx+N(x,y)dy=0$$ then $phi$ would satisfies $$d(\phi(x,y))$$ $\phi(x,y)+C$ satisfies the same condition: $$d(\phi )x,y)+C) =d(\phi(x,y))+d(C)=0$$ Hence, $\phi(x,y)+C$ is also potential function of the given differential equation.