Translate this sentence into an equation. *look at picture* !

(insert any random letter)-(4x8)=9
Answer is 23 for the random number

The difference of a number times 4 and 8 is equal to 9

ok starting with number- this is the variable which we will call "n"

The difference of a "n" times 4 and 8 is equal to 9

now looking at difference- which means subtract, you need to determine which 2 numbers you need to subtract which can be determined by looking for an "and"

The difference of a "n" times 4 and 8 is equal to 9
so grouping up the relevant information
("n" times 4) - 8 is equal to 9

well "is equal to is the equivalent of an equal sign
 ("n" times 4) - 8 is equal to 9
 ("n" times 4) - 8 = 9

now, "n" times 4 is just n*4 or 4n
replacing that

4n -8 =9

tada

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