Answer
Please see the work below.
Work Step by Step
According to law of conservation of energy
$mgh=\frac{1}{2}Kx^2$
This simplifies to:
$K=\frac{2mgh}{x^2}$
We plug in the known values to obtain:
$K=\frac{(2)(0.065)(9.8)(35)}{(0.14)^2}=2300\frac{N}{m}$
