To solve the problem it is necessary to use the concepts related to the calculation of periods by means of a spring constant.
We know that by Hooke's law
[tex]F=kx[/tex]
Where,
k = Spring constant
x = Displacement
Re-arrange to find k,
[tex]k= \frac{F}{x}[/tex]
[tex]k= \frac{mg}{x}[/tex]
[tex]k= \frac{(0.35)(9.8)}{12*10^{-2}}[/tex]
[tex]k = 28.58N/m[/tex]
Perioricity in an elastic body is defined by
[tex]T = 2\pi \sqrt{\frac{m}{k}}[/tex]
Where,
m = Mass
k = Spring constant
[tex]T = 2\pi \sqrt{\frac{0.35}{28.58}}[/tex]
[tex]T = 0.685s[/tex]
Therefore the period of the oscillations is 0.685s