String fixed at both ends is vibrating at frequency of its 2nd harmonic. How many antinodes & nodes are there?

since it is vibrating at its second harmonic, that means that the wavelength of the string's vibration is 1/2 of the natural harmonic, which means the frequency is doubled. in the natural vibration, you would have 2 nodes and 1 antinode. So, in the 2nd harmonic you would have 3 nodes and 2 antinodes.