A charge $Q_{0}$ is at the origin. A second charge, $Q_{x}=2 Q_{0},$ is brought from infinity to the point $x=a, y=0 .$ Then a third charge $Q_{y}$ is brought from infinity to $x=0, y=a$. If it takes twice as much work to bring in $Q_{y}$ as it did $Q_{x}$, what's $Q_{y}$ in terms of $Q_{0} ?$