FOURIER TRANSFORM: How can I find the index of data points

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I am a senior in high school and am currently trying to conduct an exploration on Fourier Analysis, specifically using the Discrete Fourier Transform to analyze a chord played on my piano. Basically, I'd play a chord of 3 notes, deduce the different frequencies of the sound wave, and estimate the chord played. According to the formula below, I have recorded the sound wave of Amplitude-time domain however I do not understand what is meant by the "index of data points" and how to find it from this graph. Is there any way someone could help with that as I am a bit unfamiliar to this. Thanks. This is the DFT formula and the index of data points fj