What is equal to X?

Answer 1

In an equation "x" is the value assigned to one of the unknown quantities in the equation. (example: y= mx+b).

X is often used to describe any unknown thing, as in x-ray which were named this because the nature of the ray was initially unknown.

Answer 2

If you're talking about Roman Numerals, 10. If you're talking about something else, it's unknown; that's why they used a variable to represent it. (It may be possible to find the value of the variable, but you'd have to give a lot more details.)

Answer 3

"X" as well as other letters are used in algebra as a variable, meaning either that they can have any value, or that their value is initially unknown.