see link 1 below under related questions.
a very similar problem was answered.
A 210 g air-track glider is attached to a spring. The glider is pushed in 12.0 cm against the spring, then released. A student with a stopwatch finds that 13 oscillations take 16.0 s. Find the spring constant k.
2 answers
T = 16 s/ 13 = 1.23 s
T = 2pi sq rt (m/k)
1.23 = 2 pi sq rt (.21 g/k)
k = 5.473
T = 2pi sq rt (m/k)
1.23 = 2 pi sq rt (.21 g/k)
k = 5.473