Question

In maths for every problem why will they use term x and y?

Answer 1

They are variables that don't have any definitive value and are usually used when we dont know the value of a specific thing. (common in algebraic equations)
Hope this clears your frustrating doubt

Answer 2

because of they suppose x & y .
it is not a matter that you are supposing same U can use other letter of alphabet but method of solving problems should same U will get right answer in ur way.