Answer
$7.2\times10^{14}Hz$
Work Step by Step
At the minimum frequency, the ejected electrons have no kinetic energy. The energy of a photon equals the work function, according to equation 27–5a.
$$KE=hf_{min}-W_o=0$$
$$f_{min}=\frac{W_o}{h}=\frac{4.8\times10^{-19}J}{6.63\times10^{-34}J \cdot s }$$
$$=7.2\times10^{14}Hz$$
