Answer:
k = 25.18 N/m
Step-by-step explanation:
Simple Harmonic Oscillator
It consists of a weight attached to one end of a spring being allowed to move forth and back.
If m is the mass of the weight and k is the constant of the spring, the period of the oscillation is given by:

If the period is known, we can find the value of the constant by solving for k:

Substituting the given values m=5 Kg and T=2.8 seconds:

k = 25.18 N/m