Answer:
81 books
Step-by-step explanation:
If you divide 731 into 9 you'll get 81.2
so that means you have 2 books left over that wont be on the shelf
so you'll have to round to 81
Equations with absolute value:

Where k is a positive number; if k is a negative number, the equation is impossible (absolute value is always positive).
How to solve:


Then:
1. |x+7|=12
x+7=12 V -x-7=12
x=5 V -x=19
x=5 V x=-19
{-19, 5}
2. |2x+4|=8
2x+4=8 V -2x-4=8
2x=4 V -2x=12
x=2 V x=-6
3. 3|3k|=27
3×3k=27 V 3×(-3k)=27
9k=27 V -9k=27
k=3 V k=-3
{-3, 3}
4. 5|b+8|=30
5×(b+8)=30 V 5×(-b-8)=30
5b+40=30 V -5b-40=30
5b=-10 V -5b=70
b=-2 V b=-14
{-14, -2}
5. |m+9|=5
m+9=5 V -m-9=5
m+9=5 V m+9=-5
It is 4 1/2. If you keep the first number switch the division symbol into a multiplication sign and find the reciprocal of the last number you would get this.
26.80 x .15 gives you the answer, which is $4.02 in tip
The another way to represent the same line, for solving
will be: 
<u><em>Explanation</em></u>
Given equation of the line is: 
"Solving
" means <u>we need to get</u>
<u>alone in left side.</u> So, we will eliminate all other terms except
from the left side.
First we will <u>subtract 2x </u>from both sides. So, we will get......

Now, we will <u>divide both sides by 3</u> for getting
alone. So.....

Thus, the another way to represent the same line, for solving
will be: 