Answer:
The first harmonics are the fundamental, the first overtone, and the second overtone,
For a string fixed at both ends you have:
N-A-N F/2
N-A-N-A-N F
N-A-N-A-N-A-N 3 F/2 each harmonic increases by F/2
Where N are the anti-nodes and A the antinodes
N-A-N-A-N represents 1 wavelength for string fixed at both ends
So 1 wavelength is 64 cm
280 m/s = F * .64 m and F = 437.5 Hz (second harmonic)
First harmonic = F/2 = 218.75 / sec
Third harmonic = 3 F / 2 = 656.25 /sec