Answer:
5.9 cm
Step-by-step explanation:
f: frequency of oscillation
frequency of oscillationk: spring constant
frequency of oscillationk: spring constantm: the mass

in this problem we know,
F= 1.4 Hz
m= 0.26 kg
By re-arranging the formula we get

The restoring force of the spring is:
F= kx
where
F= 1.2 N
k= 20.1 N/m
x: the displacement of the block
