Answer
Spring Constant $K= 89N/cm$.
Work Step by Step
We know that
$P.E_{spring}=\frac{1}{2}Kx^2$
Putting the values, we get
$25=\frac{1}{2}K(.075)^2$
$2\times25=k(.005625)$
$\frac{50}{.005625}=K$
or
$K=8888.9$
$K=8.9\times10^3\frac{N}{m}$
As $1m=100cm$
Hence
$K=8.9\times10^3\frac{N}{100cm}$
$K=89\frac{N}{cm}$
