jmc

If the point $(a,b)$ is reflected over the $y$-axis, it will land on the point $(-a,b)$. Thus, $j=-a$ and $k=b$. We were given that $a+j=0$, and $a+(-a)=0$ so this is satisfied. From $b+k=0$, we find $$b+(b)=0\Rightarrow 2b=0$$ $$\boxed{b=0}$$