Two fixed point charges are positioned on the \(x\)-axis: charge \(q_1 = +3Q\) at \(x = -d\) and charge \(q_2 = -Q\) at \(x = +d\). A third point charge \(+q\) is initially located at the origin \(x = 0\). An external agent slowly moves charge \(+q\) from the origin to point \(P\) located at \(x = +3d\). Which of the following expressions represents the work done by the external agent on charge \(+q\) during this process?