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)