Confused physics question-please help

Question

On your first trip to Planet X you happen to take along a 150 g mass, a 40-cmcm-long spring, a meter stick, and a stopwatch. You're curious about the acceleration due to gravity on Planet X, where ordinary tasks seem easier than on earth, but you can't find this information in your Visitor's Guide. One night you suspend the spring from the ceiling in your room and hang the mass from it. You find that the mass stretches the spring by 22.7 cm . You then pull the mass down 10.5 cm and release it. With the stopwatch you find that 11.0 oscillations take 17.3 s 

Can you now satisfy your curiosity? Express in m/s^2

Answer 1

mg = k·Δl; t/n = 2π√m/k; k = mg/Δl = 4π²mn²/t²; g = 4π²n²Δl/t² = 4π²·11²·0.227m/17.3²s² = 3.6231 m/s²