A block of mass \(M\) on a frictionless horizontal surface is held against a non-ideal horizontal spring, compressing it by a distance \(D\) from its equilibrium length. The magnitude of the restoring force exerted by the spring when compressed by a displacement of magnitude \(x\) is given by \(F(x) = kx + \beta x^2\), where \(k\) and \(\beta\) are positive constants. The block is released from rest and loses contact with the spring at the instant the spring returns to its equilibrium length. What is the speed of the block at the moment it leaves the spring, in terms of \(M\), \(D\), \(k\), and \(\beta\)?