
Answer:
M = 175 kg
Explanation:
In the resolution of the harmonic oscillator movement of a system and a mass with a spring, the angular velocity is
  w = √ k / m
Where k is the spring constant and m the mass
In this case the mass is the mass of the chair (m) plus the mass of the astronaut (M)
  M all = m + M
The angular velocity and the period are related by
  w = 2π / T
Substituting
  2π / T = √(k/(m + M))
We calculate the astronaut's mass
  4π² / T² = k / (m + M)
  M = k T² / 4π² - m
  M = 569 3.6² /(4π²) - 11
  M = 186.8 - 11
  M = 175 kg