KöMaL 2016 B4815

The binary operation $\circ$ is defined on the set of integers. Given that $a\circ(b+c)=b\circ a+ c\circ a.$ for all integers $a$, $b$ and $c$, show that there exists an integer $k$ such that $a\circ b= k\cdot a \cdot b$ for all $a$ and $b$.