Answer
$\text{Spring constant}$ ($k$) $=180N/m$
Work Step by Step
$\text{In simple harmonic motion, restoring force is given by:}$
$$F = -kx$$
$x\text{ is the displacement from mean position}$
$k\text{ is the spring constant}$
$\text{Here "-" sign denotes direction opposite to the displacement x}$
$\text{Also according to Newtons Second law:}$
$$F = ma$$
$\text{Comparing the magnitudes:}$
$$kx = ma$$
$$k = \frac{ma}{x} = \frac{0.42\times 9}{2.1\times 10^{-2}} = 180N/m$$
