What is the longest wavelength of light that will emit electrons from a metal whose work function is?

 Best   hf = work function + KE 
However if you are looking at the max wavelength (or threshold frequency) then there will only be just enough energy for the photoelectrons to be liberated, hence their KE will be 0. 
So hf = work function 
convert eV to joules, 2.4 x (1.6 x 10^-19) = 3.84 x 10^-19 
therefore, hf = 3.84 x 10^-19 
f = 3.84 x 10^-19 / planck's constant which is 6.63 x 10^-34 
f = 5.79 x 10^14 Hz 
c = frequency x wavelength, 
wavelength = speed of light/frequency 
= (3x10^8)/(5.79x10^14) 
=5.18 x 10^-7 metres

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