When it is more effective to use a general letter x rather than a concrete numeral to represent a number, whose value constantly varies—an economic way to convey the idea of a variable in an equation or a mathematical sentence.
Why an X, and not an A, B, or C is because the letters x, y, and z have traditionally been used to stand for unknowns, while the letters a, b, and c for constants.
by MathPlus November 18, 2018