Since he does 8 periods in 40 seconds 1 period takes 40s/8=5s, lets call this T. From formula T=2pii*sqrt(m/k) we can solve k (spring's stiffness constant). k=m/(T^2/(2pii)^2), k=100,275...N/m.
Then F=xk, where F is the stretching force caused by the jumper, F=mg, 63,5kg*9,81m/s^2=622,935N. x is the amount the thing has stretched so x=F/k, x=622,935N/100,275...N/m=6,2122m.
Thereby the cord's original lenght is 28,5m-6,2122...m=22,2877...m.
Correctly rounded ansvers k=100N/m and unstretched lenght 22,3m.