Answer
$T=0.28s$
Work Step by Step
We know that
$T=2\pi\sqrt\frac{m}{K}$..........................eq(1)
Also,
$K=\frac{F}{x}=\frac{75}{0.30}=250\frac{N}{m}$
We plug in the known values in eq(1) to obtain:
$T=2\pi\sqrt\frac{0.50}{250}=0.28s$