Answer: 864.36N/m
Explanation:
Using formula for the amplitude
T =2ΒΆ(m/k)^0.5
T is the amplitude
M is the mass
K is the force constant
It becomes;
1.30 = 2* 3.142* (35.5/k)^0.5
Solving algebraically gives
K = 29.4^2
K = 864.36 N/m
NB: this is calculated from the period and mass of the chair alone.