Use the blackboard. When first introducing the concept of evaluating mathematical expressions, all of your students will need to see some examples of how it's done. The blackboard is a great teaching aid that can show all of your students at once.
Write an algebra expression on the board. Be sure there are letters and numbers, such as 5x -- x + y.
Show your students what the letters stand for. To evaluate the expression, you need to replace the letters with numbers. In this example, let x equal 2 and y equal 7.
Rewrite the expression. In this example, the equation will now read 5 (2) + 2 -- 7.
Solve the expression. At this point, ask your students to help you. Do the multiplication first, 5 times 2 gets 10. Then, add 2 and subtract 7 to get 15.
Ask your students if they have any questions about the process of evaluating the expression.
Write some more examples on the board. Ask a few students to come up and complete the problems. Get volunteers if possible. Once the expressions are evaluated by your students, ask again if there are any questions.
Reinforce the lessons with an independent worksheet assignment and a homework assignment. Go over each during class time.