Calculating the Spring Constant.

A bungy cord 20m in length has a 85kg ball attached to it. The ball is dropped off a 45m building and stops 2m above the ground before going back up.

What is the spring constant?

I attempted to use $$U=\frac{1}{2}kx_{max}^2$$ rearranged for k $$k=\frac{2U}{x_{max}^2}$$ with x being the distance from equilibrium, or half the the total length $x_{max}=\frac{1}{2} 43m=21.5m$
then conserving energy I assumed $U=mgh$ at the top of the building with $h=45m-2m=43m$.

From all this I got a spring constant of $k=155.135 \frac{N}{m}$

Question

Calculating the Spring Constant. 

A bungy cord 20m in length has a 85kg ball attached to it. The ball is dropped off a 45m building and stops 2m above the ground before going back up. 

What is the spring constant?

I attempted to use $$U=\frac{1}{2}kx_{max}^2$$ rearranged for k $$k=\frac{2U}{x_{max}^2}$$ with x being the distance from equilibrium, or half the the total length $x_{max}=\frac{1}{2} 43m=21.5m$
then conserving energy I assumed $U=mgh$ at the top of the building with $h=45m-2m=43m$.

From all this I got a spring constant of $k=155.135 \frac{N}{m}$