Why do blocks have the same velocity at maximum spring compression?

[Nikstykal]

Homework Statement

The masses of blocks A and B are 2 kg and 3 kg, respectively. Block A slides to the right on the smooth surface and strikes the spring attached to stationary block B. The spring deforms during the collision process. What quantity do the blocks have in common at the instant the spring has the maximum compression?

The answer is velocity and I'm unsure how.

Homework Equations

mv1 + ∫ΣFdt = mv2

The Attempt at a Solution

 

[haruspex]

If the velocities are not the same, what is happening to the length of the spring?

 

[Nikstykal]

You slick guy you... compressing or extending. Makes sense now. Thanks.