The mass of the block. m=0.6 kg
The displacement of the spring, x=0.15 m
Total force applied on the spring from the mass is given by,

Where g is the acceleration due to gravity.
The restoring force of the spring is given by,

Where k is the spring constant
Thus, from equation (i) and equation (ii),

On substituting the known values,

Thus the force constant of the spring is 39.8 N/m