What does "c ≠ 0" mean? I've heard the "≠" sign means "does not equal." Does this mean that c ≠ 0 means that "(variable) does not equal zero"? What is it's usage?

Answer 1

yes, it means "not equal"

suppose you had
y = 4/(x-5) and you wanted the domain of the function

that is, what values of x could I use ??

Well, we could use any real number for x, except 5

and we would write it as:
x = any real number, x ≠ 5