Question

I want to know that how we get understood that in which place or where we have to use "let be X"... OR We Assume " X"

Answer 1

It will be given in your question that what is really needed for solution and it's value is not given so you can assume it be x

Answer 2

If you are proving an if-then statement, i.e. If X then Y, then you assume X and prove Y. There is a possibility that X is never true, but that doesn't matter as long as Y is true whenever X is true.