A block of mass 2M rests on a horizontal, frictionless table and is attached to a relaxed spring. The spring is nonlinear and exerts a force F(x)=-Bx³, where B is a positive constant and x is the displacement from equiLiBrium for the spring. A block of mass 3M and initial speed v₀ is moving to the left. The two blocks collide and stick to each other. The two-block system then compresses the spring a maximum distance D. Determine an expression for the kinetic energy of the two-block system immediately after the collision.