I have this equation: 
cpd = 2/(1 + exp(-0.004*200)) - 1

i want it to give me a new equation for cp (not solving the equation). 
what i've tried:

2/(1 + exp(-0.004*200)) - 1 = cpd = 0.379948962
q1 := 0.379948 = 2/(1 + exp(-0.004*cp)) - 1
isolate(q1, cp) = cp = 199.99 (this is the answer but i want the equation)

if this is not clear enough then i will give a simpe example: 

given equation: x-2 = 5
WHAT I DON'T WANT: x = 3

WHAT I WANT: x=5-2