Three point charges are fixed at the vertices of an equilateral triangle of side length \(d\). Two of the vertices each hold a charge of \(+q\), and the third vertex holds a charge of \(-q\). A fourth particle with charge \(+q\) is brought from infinitely far away to point \(P\), which is located at the midpoint of the line segment connecting the two \(+q\) charges. Which of the following expressions represents the work done by an external agent to bring the fourth particle to point \(P\)?