What do you call replacing a variable in an expression by a number?

A variable is a letter that represents a number. An expression that contains at least one variable is called variable expression, also called algebraic expression. A variable expression has one or more terms. A term is a number, a variable, or a product of numbers and variables. For example,

3(x^2)y + 2xy + x - 7 is a variable expression, where you have 4 terms.

When working with variable expression, you often use the substitution principle:

If a = b, then a may be replaced by b in any expression.

The set of numbers that a variable may be represent is called replacement set, or domain, of the variable. To evaluate a variable expression, you replace each variable with one of its values and simplify the numerical expression that results.

Example: Evaluate the expression 2x - 4y for x = 5 and y = -9.

Solution:

2x - 4y

= 2(5) - 4(-9)

= 10 + 36

=46