## Evaluating a 2-Variable Expression

### Evaluate Expressions: Learn

A mathematic *expression* is a group of numbers and
operators which can be evaluated to determine their value.

There may be *variables* in the expression which
represent numbers that may or may not be known. A variable
is simply a symbol used as a placeholder for the unknown
number.

If we know the values of the variables, we can find the value of the entire expression.

For example, if x=3 and y=5, then the expression 7x+y-4 can be evaluated in the following way:

7x+y-4 =7*3+5-4 =21+5-4 =22

The very first step where the variables are replaced by their values
is called *substitution*.

### Evaluate Expressions: Practice

#### Evaluate using the given value.

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If x= and y=, then

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