Finding the Spring Constant for Bungee Jumping

[wangking]

Homework Statement

Hey, i need some help here
I'm trying to figure out the spring constant for bungee jump

Say, I'm 735N and the rope will apparently double in length when i jump off it.
I don't know the length of the rope thought.
but i do know it does have a spring constant.
we are trying to find its N/m ratio.
How to find out is the spring constant using the Hooke's Law F=-KX

Please help :)

Homework Equations

F=-KX
EPE=1/2KX^2
EPE=GPE
mgh=1/2kx^2

The Attempt at a Solution

i can't find the spring constant

 

[rl.bhat]

Fall of potential - mgx.
PE stored in the spring is 1/2kx^2
-mgx = 1/2kx^2
2mg = -kx