whenever the things contained by the sin function = a factor of 2pi , a complete cycle is made.
so in this case, whenever 2pi/7x = 2pi , a complete cycle is made.
2pi/7x = 2pi
1/7x = 2pi/2pi
1/7x = 1
7x = 1
x = 1/7
therefore, the period is x = 1/7 for every cycle
