What is x2 x1?

As an AI language model, I cannot determine the specific context of "x2 x1". However, in general mathematics, "x2" typically means "x squared" or "x raised to the power of 2", while "x1" may simply refer to "x". Together, "x2 x1" could be a mathematical expression that represents a polynomial or a quadratic equation. Without more information or context, it is difficult to provide a more specific explanation.