Answer:
[tex]\large \boxed{\text{(a) 7.800 Hz; (b) 20.3 m/s; 40.6 m/s; 60.8 m/s}}[/tex]
Explanation:
a) Fundamental frequency
A harmonic is an integral multiple of the fundamental frequency.
[tex]\dfrac{\text{23.40 Hz}}{\text{15.60 Hz}} = \dfrac{1.500}{1} \approx \dfrac{3}{2}[/tex]
[tex]f = \dfrac{\text{24.30 Hz}}{3} = \textbf{7.800 Hz}[/tex]
b) Wave speed
(i) Calculate the wavelength
In a  fundamental vibration, the length of the string is half the wavelength.
[tex]\begin{array}{rcl}L & = & \dfrac{\lambda}{2}\\\\\text{1.30 m} & = & \dfrac{\lambda}{2}\\\\\lambda & = & \text{2.60 m}\\\end{array}[/tex]
(b) Calculate the speed
s
[tex]\begin{array}{rcl}v_{1}& = & f_{1}\lambda\\& = & \text{7.800 s}^{-1} \times \text{2.60 m}\\& = & \textbf{20.3 m/s}\\\end{array}[/tex]
[tex]\begin{array}{rcl}v_{2}& = & f_{2}\lambda\\& = & \text{15.60 s}^{-1} \times \text{2.60 m}\\& = & \textbf{40.6 m/s}\\\end{array}[/tex]
[tex]\begin{array}{rcl}v_{3}& = & f_{3}\lambda\\& = & \text{23.40 s}^{-1} \times \text{2.60 m}\\& = & \textbf{60.8 m/s}\\\end{array}[/tex]
