Whic describe the intersection of line m and line n

The graph shows the distance y, in inches, a pendulum moves to the right (positive displacement) and to the left (negative displacement), for a given number of seconds x. how many seconds are required for the pendulum to move from its resting position and return? enter your answer in the box.

Answer for 10 points show work you guys in advanced < 3

Can someone explain to me how to do