Two carts of masses \(m\) and \(2m\) are held together at rest on a rough horizontal surface with a compressed spring of negligible mass between them. At time \(t = 0\), the spring is released, pushing the cart of mass \(m\) in the \(+x\)-direction and the cart of mass \(2m\) in the \(-x\)-direction. The coefficient of kinetic friction between each cart and the surface is \(\mu\). Assuming both carts are still sliding, what is the velocity \(v_{\text{cm}}\) of the center of mass of the two-cart system in the \(+x\)-direction as a function of time \(t\)?