KöMaL 2015 K462

$a)$ $f$ is a function defined on the set of real numbers. Given that $f(a)-f(b)=f(a\cdot b)$ for all $a$ and $b$ , find the value of $f(2015)$. $b)$ Is there a function $g$ defined on the set of real numbers such that $g(a) - g(b) = 2\cdot g(a\cdot b) - 2$ for all $a$ and $b$ ?