What symbal is used to represent unknowm numbers?

Answer 1

Such a symbol is called a "variable". It can be a letter, a group of letters (especially in computer programming), or a letter with a subscript (a lowered number, or some other letter - this is more common in math). The letters need not be from the Latin alphabet; Greek letters, or sometimes Hebrew letters, are also used.

Answer 2

The symbol (not symbal) can be anything you like. Letters from the Roman alphabet are most common but those from the Greek or Hebrew alphabet are also used.