To solve this problem we will apply Hooke's law. Which says that the force on a spring is equivalent to the product between the elastic constant and the displacement of the spring.

Here,
k = Spring constant
x = Displacement
Rearranging to find the spring constant,

Spring compresses it to 29 cm so

The force in this case is equivalent to the weight of the spring


Therefore the spring constant is 215.6N/m