So sorry! As my last comment I feel as if working with the elastic potential energy equation
For springs: PE = (1/2)kx^(2) is worth a shot. Since PE Is also equal to mgh or mgx in this case. Keep on working hard! You got this!
well for (a), please forgive me if I am wrong, but I am visualizing the problem as if you placed the 3kg block on top of the 5kg spring system. Therefore, the original length was when the 5kg spring system had no extra weight and the compressed length was when you placed the 3kg block on top...