In the dangerous "sport" of bungee-jumping, a daring student jumps from a hot-air balloon with a specially designed elastic cord attached to his waist, as shown in the figure below. The unstretched length of the cord is 26.0 m, the student weighs 675 N, and the balloon is 36.0 m above the surface of a river below. Calculate the required force constant of the cord if the student is to stop safely 4.00 m above the river.