Part of the river she observed fell 1.68 millimeters per year for 1.5 years. use the distributive property to calculate the exact variation in water level for the time period.

1.68 mm per year for 1.5 years could be rewritten as 
(2.00-0.32) mm per year for 1.5 yrs.

Multiply:  (1.5 yrs)(2.00-0.32)(mm/yr) = 3 mm + 0.48 mm = 3.48 mm

This approach makes use of the distributive property of multiplication.