c := 299792458
k := 1.3806503*(10^(-23))
h := 6.626068*(10^(-34))
B[x_, 
        T_] := ((2*h*(x^3))/((c^2)*((E^(((h*x)/(k*T))))  - 1)))

Plot[{B[x, 3000], B[x, 4000], B[x, 5000], B[x, 6000]}, {x, 1, 
        (10^15)}]

B2[y_] := ((y^3)/((E^y) - 1))

(∂_y) B2[y]

FindRoot[((3 (y^2))/(-1 + (E^y))) - (((E^y) (y^3))/(((-1 + (E^y)))^2)) == 
        0,  {y, 1, 100}]

(∂_x) B[x, T]

Simplify[%]

Plot[{3*(1 - (E^(-y))), y}, {y, 1, 3}]

y := 2.8214393721220787`

    a = ((k*y)/h)