Answer:
x = 0.06 m = 6 cm
Step-by-step explanation:
extension (x) = 3 cm = 0.03 m
force = mg = kx
k (spring constant) = mg / x = mg / 0.03
loss in potential energy = gain in potential energy
mgx = o.5k

mg = 0.5kx
x = mg / 0.5k
recall that k = mg / 0.03
therefore
x =

x = mg x

x =

x = 0.06 m = 6 cm