Final answer:
Ericka should multiply the average annual rise in water level (1.8 mm/year) by the time period (6.2 years) to get the expected increase, then subtract the amount the water level actually fell to find the difference.
Step-by-step explanation:
The student, Ericka, is attempting to compare her observation of water levels falling with the average annual trend of water levels rising. To do this, she needs to calculate the total expected increase over her observation period of 6.2 years based on the average annual rise of 1.8 mm/year and then compare it with her actual observation of the water level change. To find the difference between the average annual trend and her observation, Ericka should multiply the average annual rise by the time period and then subtract her observation of how much the water level fell.If the water level was supposed to rise by 1.8 mm each year, in 6.2 years, it was expected to rise 1.8 mm/year * 6.2 years = 11.16 mm. If Ericka observed a fall in water level, let's say 'x' mm, then the difference would be the expected rise minus the observed fall: 11.16 mm - x mm.Conclusion: The correct method Ericka should use is to calculate the product of the average annual trend and the time period, then subtract her observation from this product. This would give Ericka the difference between the expected rise in water levels based on the average trend and the fall in water levels that she actually observed.