Question

explain why it does not matter what letter or symbol is used to find an unknown number

Answer 1

a b c d... and x are all representing 1 unknown #. The particular letter
is not revelent unless its like f (x) =.... which just means y but g(x) sometimes means graph.
otherwise it doesn't matter at all

Answer 2

It does not matter so long as it is consistent and not repeated for a different meaning