Two thin, concentric, conducting spherical shells of radii \(R\) and \(2R\) carry total charges \(+Q\) and \(+2Q\), respectively. A point charge \(+q\) is moved from an initial position at distance \(r = \dfrac{1}{2}R\) from the center to a final position at distance \(r = 3R\) from the center. Which of the following expressions represents the change in electric potential energy \(\Delta U = U_f - U_i\) of the system consisting of the shells and the point charge?