Verrry interesting !
In the fraction at the end of the equation ... c/ac ... the numerator and denominator can both be divided by 'c', and the fraction becomes 1/a .
Then the equation becomes d = a + b + 1/a
The equation can't be solved for 'c', because there is no relationship between 'c' and any of the other terms. 'c' can be anything at all, and it has no effect on anything else in the equation.
Or, to put a finer point on it, 'c' is not a function of 'a', 'b', or 'd' .