RE: How X became the Alphabet in Unknown Mathematical Equation

So the origin of X can be traced but cannot be confirmed. X has really become a big deal in our world. I remember in high school, there was a time I used Q as my unknown and my teacher failed me. He made it clear to me that X is the major unknown and in rare cases when X is known, Y becomes the unknown. It became more serious with calculus but all the same, I still wonder till date why he didn't just take Q as the unknown instead of making a whole scene out of ordinary letters.