a roofer props a 24 foot ladder against a wall so that the top of the ladder reaches the roof that needs repair. If the angle of elevation from the bottom of the ladder to the roof is 75 degrees, how far is the ladder from the base of the wall?​

In the area that she observed, the water level fell 13.64mm over 6.4 years, for an average annual rate of change of:
-13.64 ÷ 6.2 = -2.2 mm/ year. 

The difference between how much average water levels rose and how much water levels in her part of the river fell is:
1.8 -(-2.2) = 4 mm/year

During the 6.2 year period: 
4 mm/year * 6.2 = 24.8 mm