answer
88.2 N/m
set up equation
when the block drops to 5.0 cm, the weight of block and the spring force on the block are equal because there is no acceleration on the block (at rest)
so mg = Fs
due to Hooke's law, Fs = kx
where k is the spring constant and x is the displacement
mg = Fs = kx
mg = kx
values
convert given values to standard units
m = 0.450 kg
g = 9.8 m/s^2
x = 5.0 cm = 0.05 m
plug in values and solve
mg = kx
0.45 * 9.8 = 0.05x
x = 0.45 * 9.8 / 0.05
x = 88.2 N/m