A block on a horizontal surface of negligible friction is placed in contact with an ideal spring, as shown to the right. The block is moved to the left so that the spring is compressed a distance x from equilibrium and then released from rest. The block has a speed of vi when it separates from the spring. When the spring is compressed a distance of $2$x and the block is released from rest, the velocity of the block when it separates from the spring will be ?