The relative luminosity for a wavelength of 600 nm is 0.6 ....

Luminous flux is given by the product of a constant, relative luminosity, and radiant flux.

For the same brightness sensation, the luminous flux must be equal for both wavelengths.

Let the radiant flux at $600 \text{ nm}$ be $\Phi$.

The relative luminosity at $555 \text{ nm}$ is $1$ (maximum).

Equating the luminous flux for both wavelengths:

$V(600 \text{ nm}) \times \Phi = V(555 \text{ nm}) \times 120$

$0.6 \times \Phi = 1 \times 120$

$\Phi = \dfrac{120}{0.6} = 200 \text{ W}$