The stopping potential for photoelectrons emitted from a surface illuminated by light of wavelength 3315 Å is 0.21 V. If the threshold frequency is   $x$  × 10¹³ / s, where  $x$ is ____

(Given, speed light = 3 × 10⁸ m/s, Planck’s constant = 6.63 × 10^–34 Js)

[Round off to two decimal places]