Lets check
[tex]\\ \rm\rightarrowtail T=2\pi \sqrt{\dfrac{m}{k}}[/tex]
[tex]\\ \rm\rightarrowtail 0.5=2\pi\sqrt{\dfrac{0.015}{k}}[/tex]
[tex]\\ \rm\rightarrowtail 0.25=4\pi^2 {\dfrac{0.015}{k}}[/tex]
[tex]\\ \rm\rightarrowtail k=\dfrac{4(3.14)^2(0.015)}{0.25}[/tex]
[tex]\\ \rm\rightarrowtail k=\dfrac{0.5916}{0.25}[/tex]
[tex]\\ \rm\rightarrowtail k=\dfrac{0.591576}{0.25}[/tex]
[tex]\\ \rm\rightarrowtail k\approx 2.37N/m[/tex]