Answer
$m=0.325Kg$
Work Step by Step
$'\omega'$ angular frequency and $'m'$ are related as follows
$\omega=\sqrt\frac{K}{m}$
squaring both sides,we get
$\omega^2=\frac{K}{m}$
$m=\frac{K}{\omega^2}$
put the values
$m=\frac{400}{(35.07)^2}$
$m=0.325Kg$
