Maybe this is why I'm still in high school.
Just ignore me then... I haven't learnt those things yet.
c is and element of a real number...
Isn't -c a real number? Why isn't -c allowed?
Since C is an arbitrary real number, it doesn't matter whether C is positive or negative and it doesn't matter whether we call it "C" or "- C". The same thing happens often with exponentials. If you have a solution to an equation of the form where C is an arbitrary constant, you can write that as or simply as . Strictly speaking, we should use a different symbol, say C' with the explanation that but typically, knowing that they are both just arbitrary numbers, that is not done.