What is the damping coefficient of a simple spring mass system

Question

1. In a spring-mass system, the mass is 0.1kg and the stiffness of the spring is 1kN/m, by introducing a damper, the frequency of oscillation is found to be 90% of the original value. What is the damping coefficient of the damper?                                                                                                                                                                

(a) 1.2 N.s/m                                                       (b) 3.4 N.s/m

(c) 8.7 N.s/m                                                       (d) 12.0N.s/m