What is 21 x -3?

21x - 3 is an algebraic expression that consists of two terms: 21x and -3. The term 21x is made up of the coefficient (21) and the variable (x), while -3 is simply a constant. Expressions of this type can be simplified by combining like terms. However, since 21x and -3 are already not like terms (they have different variables or no variables at all), this expression cannot be further simplified.