Question 18
Find all functions $f$ defined on $\mathbb{R}$, taking values in $\mathbb{R}$ so that
$$
f(f(x - y)) = f(x)f(y) - f(x) + f(y) - xy
$$
for all real numbers $x, y$.
$$
f(f(x - y)) = f(x)f(y) - f(x) + f(y) - xy
$$
for all real numbers $x, y$.