The stopping potential for photoelectrons emitted from a surface illuminated by light of wavelength 6630 $$\mathop A\limits^o $$ is 0.42 V. If the threshold frequency is x $$\times$$ 10¹³ /s, where x is _________ (nearest integer).

(Given, speed light = 3 $$\times$$ 10⁸ m/s, Planck's constant = 6.63 $$\times$$ 10^$$-$$34 Js)