The threshold frequency of a metal with work function 6 . 63 eV is :

The formula to calculate the work function of the metal is given by

${\varphi }_0=h{v}_0 ...\left(1\right)$

From equation (1), it follows that

$$\begin{gathered} 6.63\times 1.6\times {10}^{-19}=6.63\times {10}^{-34}{v}_0 \\ \Rightarrow v_0=\frac{1.6\times {10}^{-19}}{{10}^{-34}} \\ \Rightarrow v_0=1.6\times {10}^{15} \mathrm{Hz} \end{gathered}$$