Answer:
10.23077 m
55.88391 N/m
Step-by-step explanation:
m = Mass of person = 58.5 kg
T = Time period =

k = Spring constant
g = Acceleration due to gravity = 9.81 m/s²
Time period is given by

The spring stiffness constant is 55.88391 N/m
The force on spring is given by

The unstreched length of the cord is 20.5-10.26923 = 10.23077 m