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For a given combination of values for the free variables, an expression may be evaluated, although for some combinations of values of the free variables, the expression may be undefined. Thus an expression represents a function whose inputs are the values assigned the free variables and whose output is the resulting value of the expression.

An expression is a combination of numbers, operators, grouping symbols (such as brackets and parentheses) and/or free variables and bound variables arranged in a meaningful way which can be evaluated. Bound variables are assigned values within the expression (they are for internal use) while free variables can take on values from outside the expression.

If an expression has more than one operation, the order in which you do the operations becomes important.

If the expression has parentheses, you should first do the operations inside parentheses.

After taking care of parentheses, you can do all multiplication and division operations.

Finally, you can do all addition and subtraction operations.

The mnemonic "Please Excuse My Dear Aunt Sally" (PEMDAS) commonly used to remember the order:

Parentheses, Exponentiation, Multiplication, Division, Addition, Subtraction.