Given,
The mass, m=35 kg
The displacement of the spring from its equilibrium position, x=65 cm=0.65 m
The magnitude of the restoring force of the spring is given by,

And the restoring force will be equal to the weight due to the mass attached to the spring.
Therefore,

Where g is the acceleration due to gravity.
On rearranging the above equation,

On substituting the known values,

Thus the value of the spring constant of the given spring is 527.7 N/m