Hi ithink I can work out the rest of teh question if someone can tell me how to use hookes law for 2 springs.
Is this just -3k(5l0-x-l0) = -3k(4l0-x). Note the l0 stands for L with a little 0 at the bottom (i.e. initial)
The spring extension of the lower spring is presumably ? Since the top spring has an equilibrium length of , the spring extension of the upper spring is . So Newton's 2nd tells us
This gives the required equation of motion.