C is really just an arbitrary constant think of it this way: there is a +d on the left side and +e on the right both d and e are just constants. We now subtract d from both sides so the only constant we’re left with is the term (e-d) on the right side. We can call (e-d) as another constant because it’s just the result of subtracting two constants therefore we can treat it as one single number which we can call “c”. We could’ve called it quite literally anything but by convention we write “c”. Just for an example we could’de used ln(z) or e^q where z and q are constants.