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a. 3.8 x 10¹⁴ s⁻¹

Explanation:

For the first line,

we have wavelength = λ = 7.9 x 10⁻⁷ m

We know that , speed of light , c is given by:

                    c = λ x ν

             Where  λ is wavelength of the line and  ν is the frequency

So , the frequency , ν = c / λ = 3 x 10⁸ / 7.9 x 10⁻⁷ = 3.79 x 10¹⁴ s⁻¹

                                                                                 ≈  3.8 x 10¹⁴ s⁻¹

Therefore the frequency is 3.8 x 10¹⁴ s⁻¹ .