h = 6.626176 10^-34

k = 1.380662 10^-23

c = 2.997925 10^8

FindRoot[ 
      1/x  ==  
        k (5 - Exp[-h c/(k x) ])/(h c),  {x, 
        0.003}]

FindRoot[ 
      x  ==  
        h c / k /(5 - Exp[-h c/(k x) ]),  {x, 
        0.003}]