The stopping potential for photoelectrons emitted from a surface illuminated by light of wavelength $6630$ Å is 0.42 V . If the threshold frequency is $x \times 10^{13} / \mathrm{s}$, where $x$ is $\_\_\_\_$ (nearest integer)
(Given, speed light $=3 \times 10^8 \mathrm{~m} / \mathrm{s}$, Planck's constant $=6.63 \times 10^{-34} \mathrm{Js}$ )