Electric Potential Energy and Work Ratio

Two identical point charges, each with charge $q$, are fixed at coordinates $(0,0)$ and $(a,0)$. A third point charge of $-q$ is placed at $(a/2,0)$ and then released. What is the ratio of the initial electric potential energy of the three-charge system to the work done by the electric field on the charge $-q$ as it moves to infinity?