So first here is the graph: http://i56.tinypic.com/b8muja.png

The question is:

The figure above shows a portion of a graph of the function f. If f(x+5)=f(x) for all values of x, then f(x)=0 for how many different values of x between 0 and 12?

the book says the answer is 9, but i do not know how i would get that answer?

So how would i get that answer? thanks