1. EXAMPLE 2 Evaluate a variable expression Substitute – 2 for x and 7.2 for y. Add the opposite – 2. = 7.2 + 2 + 6.8 Evaluate the expression y – x + 6.8 when x = – 2 and y = 7.2. y – x + 6.8 = 7.2 – (–2) + 6.8 = 16 Add.

2. One of the most extreme temperature changes in United States history occurred in Fairfield, Montana, on December 24, 1924. At noon, the temperature was 63°F. By midnight, the temperature fell to – 21°F. What was the change in temperature ? SOLUTION The change C in temperature is the difference of the temperature m at midnight and the temperature n at noon. EXAMPLE 3 Evaluate Change Temperature

3. Substitute values. Add the opposite of 63. = – 21 – 63 Write equation. Find the change in temperature. Write a verbal model. Then write an equation. STEP 1 STEP 2 = – 84 Add – 21 and – 63. Evaluate Change EXAMPLE 3 C mn =– C m – n = = – 21 + (– 63)

4. The change in temperature was – 84°F. ANSWER EXAMPLE 3 Evaluate Change

5. GUIDED PRACTICE for Example 2 and 3 Evaluate the expression when x = – 3 and y = 5.2. 4. x – y + 8 = – 0.2= – 0.2 5. y – (x – 2) = 10. 2 6. (y – 4) – x = 4.2= 4.2 A new car is valued at $15,000. One year later, the car is valued at $12,300. What is the change in value of car ? 7. Car values – $2700 ANSWER