code: def eta_mass1_to_mass2(eta, mass1, return_mass_heavier=False, force_real=True):
      
      return conversions.mass_from_knownmass_eta(mass1, eta,
          known_is_secondary=return_mass_heavier, force_real=force_real)
docs: This function takes values for eta and one component mass and returns the
      second component mass. Similar to mchirp_mass1_to_mass2 this requires
      finding the roots of a quadratic equation. Basically:
  
      eta m2^2 + (2 eta - 1)m1 m2 + \eta m1^2 = 0
  
      This has two solutions which correspond to mass1 being the heavier mass
      or it being the lighter mass. By default the value corresponding to
      mass1 > mass2 is returned. Use the return_mass_heavier kwarg to invert this
      behaviour.