Question

Grade 3: Explain why it does not matter what letter or symbol is used to find an unknown number

Answer 1

because it's just a substitute for that number that's supposed to be there

Answer 2

The coefficient (letter chosen) that is used does not matter as it is only to show that it is an unknown variable, if you have more than one variable you need more than one type of coefficient (use different letters for different ones you dont know).