Answer:
The beat frequency = 2.0 Hz
Step-by-step explanation:
From the given information
The 3rd harmonic frequency of the note is equivalent to thrice note A fundamental frequency.
i.e.

where;
note A fundamental freq.
replacing 440 Hz for



However, the 2nd harmonic of the E is equivalent to two times the fundamental frequency of the note E.
i.e.

note E fundamental freq.
replacing 659 Hz for



Finally, the beat frequency when the E string is properly tuned is:



Thus, the beat frequency = 2.0 Hz