The block of mass M moving on the friction less horizontal surface,  collides with the spring of the spring constant K and compresses it by length L.  The maximum momentum of the block after collision is

Elastic Energy stored in spring  =  12 KL2   here K is force constant, L is length of compression ∴      K.E. of the Block  E     =   12 KL2  ∴   p2   =    2M⋅E; here p = momentum, m=mass; E = KE      ∴   p   =   2ME  substitute E value p  =   2M⋅KL22  p  =   MK⋅ L