Answer
$0.47Kg$
Work Step by Step
As we know that
$K=\frac{F}{y}=\frac{mg}{y}$
$\implies K=\frac{(0.50)(9.81)}{15\times 10^{-2}}=32.7\frac{N}{m}$
We also know that
$T=2\pi \sqrt{\frac{m}{K}}$
This can be rearranged as:
$m=(\frac{T}{2\pi})^2K$
We plug in the known values to obtain:
$m=(\frac{0.75}{2\pi})^2\times (32.7)=0.47Kg$
