Two blocks are at rest on a frictionless surface. The smaller block has a mass of $m$; the larger block has a mass of $3m$.
An ideal spring, with a spring constant of $k$, is sandwiched between them. As shown in Figure 1, the spring is compressed by $x$. The blocks are released and allowed to slide apart.
Determine the final speed of the heavier block in terms of $m$, $k$, $x$, and any constants.