Question Difficulty

As shown in the figure, a block with a mass $\backslash (\backslash$mathrm$\{$m$\}=3\backslash$mathrm$\{$~kg$\}\backslash )$ is sliding without friction at velocity $\backslash (\backslash$mathrm$\{$v$\}=12\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$s$\}\backslash ).$ It undergoes a perfectly elastic collision with the bottom tip of a beam of length $\backslash ($ L$=4\backslash$mathrm$\{$~m$\}\backslash )$ and mass $\backslash (\backslash$mathrm$\{$M$\}=15\backslash$mathrm$\{$~kg$\}\backslash ).$ The beam is pivoted about a frictionless axle through its top end, and is initially hanging straight down at rest. Taking its initial velocity to be in the positive direction, what is the velocity of the block after the collision? $\backslash (\backslash$mathrm$\{$m$\}/\backslash$mathrm$\{$s$\}\backslash )$