First all numbers must be presented in the same format. So change the fractions to whole numbers.
5/8 = .633
6 5/8 = 6.633
Now you can start.
Add all the negative numbers first.
I add
(-) .633
0.4
0.4
6.0
2.9
3.2
-------------
-13.533 centimeters
Add the positive numbers together
3.2
6.633
0.3
---------
10.133
Now you can subtract
-13.533
Minus
10.133
--------
-3.4 centimeters
So the water level is down by -3.4 centimeters over the entire 9 months period.
