Answer : [tex]v = 59.02\ km/s[/tex]
Explanation : given that,
Arm length = 8.50 meters
Wavelength = 658 nm
Number of fringe = 1
We know that,
Formula of the speed is
[tex]v = c\sqrt\dfrac{N\lambda}{2L}[/tex]
Now, put the values
[tex]v = 3\times10^{8}\ m/s\sqrt\dfrac{1\times658\times10^{-9}\ m}{2\times8.50\ m}[/tex]
[tex]v = 590.21\times10^{2}\ m/s[/tex]
[tex]v = 59.02\ km/s[/tex]
Hence, this is the required solution.