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

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

${ϕ}_{0} = h{v}_{0} . . . \left(1\right)$

From equation (1), it follows that

$6 . 63 \times 1 . 6 \times {10}^{- 19} = 6 . 63 \times {10}^{- 34}{v}_{0} ⇒ {v}_{0} = \dfrac{1 . 6 \times {10}^{- 19}}{{10}^{- 34}} ⇒ {v}_{0} = 1 . 6 \times {10}^{15}Hz$