Answer:
K= 290.28 N/m
Step-by-step explanation:
Given: mass= 85 kg 
the time taken to reach point two more times in 6.8 s.
2×t= 6.8 sec
t= 6.8/2= 3.4 sec
then, the time period for oscillation is 

Here K= spring constant 
m= mass of jumper 
⇒

now plugging the values we get 

K= 290.28 N/m