f(x) =  \frac{ 2k+x }{e^x } 

k = 2 (slider from 0.1 to 2, step 0.1)
f(x) = (2 k + x) / exp(x)
text2: "f(x) = " + f
f' = Derivative(f, 1)
A = Intersect(f', xAxis, (-3.000000000031984, 0))
c: x(A)
d: f(c)
H: (c, d)
o_H(x) = exp(-x)