2.1 Algebra: Expressions (p.36-37)

Expressions
--Expressions are combinations of mathematical symbols that express a value.
I. Numerical Expression: an expression that includes only numbers and operations
Examples: 47-38 30+12+9
II. Algebraic Expression: an expression that includes a variable.
Examples: k-3 5+n 6x5xb
A. Variable: A variable is a letter or symbol that can stand for one or more numbers.
Examples: the letters "k" "n" and "b" from the algebraic expressions above are varialbes
B. Evaluate: replacing the variable with a number and then finding the value of the express
a + 150, for a = 8
replace a with 8
a + 150
8 + 150 add
158 : the value of the algebraic expression "a + 150, for a = 8" is 158

Here is an external linke with an introduction to algebra