Answer
$\lambda = 478~nm$
Work Step by Step
We can find the threshold wavelength:
$E = \frac{hc}{\lambda}$
$\lambda = \frac{hc}{E}$
$\lambda = \frac{(6.626\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{(2.60~eV)(1.6\times 10^{-19}~J/eV)}$
$\lambda = 4.78\times 10^{-7}~m$
$\lambda = 478~nm$
