The foundation of algebra is the variable, an unknown number represented by a symbol (usually a letter such as x or a). Variables can be preceded by a coefficient, which is a constant in front of the variable, such as 4x or −2a.
An algebraic expression is any sum, difference, product, or quotient of variables and numbers (such as 2x + 7y − 1). The value of an expression is found by replacing the variable with a given value and simplifying the result.