An astronaut visits a moon of Saturn, and decides to measure the accel. of gravity. He ties a rock (m=2 kg) to a rope (1.25 m), pulls the rock back 10 degrees, and releases it into simple harmonic motion. What is the accel due to gravity on the moon? I have no idea where to even start on this problem :(

Question

There was a plotted graph that shows the top of the arc is 10 degrees, and the bottom is -10 deg. and there are two waves in 10 seconds..