[เทพเวียนเกิด]

Find all functions $f:\mathbb{R} \rightarrow \mathbb{R}$ such that $\left(x+y\,\right)\left(f\left(x\,\right)-f\left(y\,\right)\,\right) = \left(x-y\,\right)f\left(x+y\,\right)
\forall x,y\in \mathbb{R} $