@Zeusex This seems like a poorly-written problem with insufficient information to me. There's no mention of 5% stretch in the problem, yet you assume this in your solution. Are you sure this is the full text of the problem? What is the significance of "after 20 m"? Rotating uncontrollably suggests that perhaps we are supposed to assume this mass is in freefall (which doesn't really make sense when attached to a spool). But even if assuming freefall, you can't just assume the height change is 4 m after 2 seconds, because you can't assume the velocity is constant at 2 m/s. Freefall means acceleration. So I don't really know what this problem is driving at. Finally, in your equation, it seems that "W" is supposed to represent weight, because you clearly substitute in mg for it in your solution. But if that's the case, then why is there a factor of 2 in the equation? The energy needing to be absorbed should just be equal to the gravitational potential energy lost, which is mgh
over the height of the fall. If you want to convert that into elastic potential energy, then it needs to be something like mgh = (1/2ks^2), but that doesn't work because we don't even know what k is here. I'd say go back to the teacher and clarify what is being asked.