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A block of mass 3kg is hung from a spring, causing it to stretch 12 cm at equilibrium, as shown above. The 3 kg block is then replaced by a 4 kg block, and the new block is released from the position shown above, at which the spring is unstretched. How far will the 4 kg block fall before its direction is reversed?

Here's my work(I'm allowed to use 10m/s^2 for gravity):

3kg(10m/s^2) + .12k = 0

.12k = 30

k = 250

250x - 4(10) = 0

x = .16 m = 16 cm

The answer I'm told is supposed to be 32... can someone explain what I'm doing wrong?