Answer
$K=147\frac{N}{m}$
Work Step by Step
We know that,
$F=Kx$
This can be written as;
$K=\frac{F}{x}$
In the given scenario: $F=mg$
So, $K=\frac{mg}{x}$
We plug in the known values to obtain:
$K=\frac{0.300(9.8)}{0.0200}=147\frac{N}{m}$