Hi Saja,
The period, by definition, is the length of a complete cycle. You can also look at it as the distance between two identical consecutive events.
Take your graph. There's a peak that happens when the x-value is 1. When does the next OR previous peak happen? Your graph shows the previous peak happening when the x-value is -2.
Now, find the distance between those two identical consecutive events (here, two consecutive peaks): 1 - (-2) = 3
So the period here equals 3.