Answer
$2.7\space MN/m$
Work Step by Step
Please see the attached image first.
Since there was no friction on the ground. To solve this problem we use the conservation of mechanical energy.
Final mechanical energy = Initial mechanical energy
$K+U=K_{0}+U_{0}$
Let's plug known values into this equation.
$0+\frac{1}{2}kx^{2}=0+mgh$
$\frac{1}{2}k(89.4\times10^{-2}m)^{2}= 41700\space kg\times9.8\space m/s\times 265\space m$
$k=2709960.5\space N/m= 2.7\space MN/m$
