Dual Nature Of Radiation

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)