Why do you use the letter x in algebra?

Answer 1

X is used as a variable to represent some unknown value in an equation. This allows us to set up a reliable means to figure out unknown values.

If you're asking why we use X instead of say, another letter, it's because X clearly doesn't look like any number, and it's easily recognizable, and easy to read.

See the differences:

3X-4=10

3Q-4=10 in this case, Q could be confused as a 0 depending on someone's 3S-4=10 handwriting. Or S could be seen as a 5.

14X+2Y=27

14J+2Y=27 in this case, J is harder to see than X.

Also, other letters stand for a lot of different stuff. J could stand for Joules, for instance. Or C could stand for Celsius.

So there isn't a clear cut reason why you use the letter X, or anything preventing you from using another letter. If you really wanted to use, say, H, go right ahead. X is just generally better.