Jon P. answered 03/11/15
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Start with the knowledge that the period of the function f(x) = sin (x) is 2π. That means that the function repeats over and over, every 2π.
Then, when you multiply x in the function definition by some number, you DIVIDE the period by that number. So for example, the period of sin (2x) = 2π / 2 = π.
In this case, you are multiplying x by 6π, so you divide 2π by 6π to get the period. The result is 1/3.