A girl with a mass of 40 kg is swinging from a rope with a length of 2.6 m. What is the frequency of her swinging?

I suspect that in your text book they derive the formula for periodic motion of a pendulum. It turns out to be:



where 'L' is the length of the rope. Note that mass does not enter into it. After you find the value for 'T' (the period of each swing in seconds) you thenn need to convert to frequency in units of cycles per second.