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)