How to solve this First Order PDE $xu_x - yu_y + yu = y$? The PDE is $xu_x-yu_y+yu=y$ .
The method of characteristics gives $\dfrac{dx}{x}=-\dfrac{dy}{y}=\dfrac{du}{y-yu}$
Then $x=-c_1y$ and thus $c_1=-\dfrac{x}{y}$ .
Then, I did $du=\dfrac{(y-yu)dy}{y}$ to try to solve for the second constant so that I can have that in terms of $f\left(-\dfrac{x}{y}\right)$ but I can't seem to do that.

$u \equiv w + 1\quad\Longrightarrow yw_{y} - xw_{x} + yw = 0$
$$
w = \phi\left(x\right)\varphi\left(y\right)
\quad\Longrightarrow\quad
y\varphi'\left(y\right) + y = x\phi'\left(x\right) = \mu = \mbox{constant} 
$$
$$
y\varphi'\left(y\right) + y = \mu
\quad\Longrightarrow\quad
\varphi'\left(y\right) = {\mu \over y} - 1
\quad\Longrightarrow\quad
\varphi\left(y\right)
=
\mu\ln\left(y\right) - y + A\,,\ A: \mbox{constant}
$$
$$
\phi'\left(x\right) = {\mu \over x} 
\quad\Longrightarrow\quad
\phi\left(x\right)
=
\mu\ln\left(x\right) + B\,,\quad B: \mbox{constant}
$$
$$
\begin{array}{|c|}\hline\\
\color{#ff0000}{\large\quad%
u
\color{#000000}{\ =\ }
1
+
\left[\vphantom{\LARGE A}\mu\ln\left(x\right) + B\right]
\left[\vphantom{\LARGE A}\mu\ln\left(y\right) - y + A\right]
\quad}
\\ \\ \hline
\end{array}
$$
