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Looking at this graph we can see that it intersects the origin, so we know we are using the sine function. Next we look at the max value of y and find it to be 4, so we know the sine function must be multiplied by 4. Next we see the function peaks at x=pi/4, we know that sin(x) normally peaks at x=pi/2. Knowing that we need our x in this function to equal pi/2, we will multiply x by 2 inside the sine function. In the end we are left with y=4sin(2x). The 4 makes the range larger, the 2 makes the oscialtions occur more frequently.
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