The algebraic expression uses the terms to denote symbols
sum of means addition
difference of means subtraction
product of means multiplication
quotient of means division
Here we have the term 6x
which actually means 6 times x or
6 multiplied by x
hence by multiplication we use the word product of
so we have the product of 6 and a number as our right answer
The confidence interval is

. This means that we can be 99% confident that the mean number of books people read lies between 9.15 and 11.85.
To find the confidence interval, we first find the z-score associated with it:
Convert 99% to a decimal: 0.99
Subtract from 1: 1-0.99=0.01
Divide by 2: 0.01/2 = 0.005
Subtract from 1: 1-0.005 = 0.995
Using a z-table (http://www.z-table.com) we see that this is associated with a z-score between 2.57 and 2.58. Since both are equally far from this value we will use 2.575.
We calculate the margin of error using

This means that the confidence interval is

The lower limit is given by 10.5-1.35 = 9.15.
The upper limit is given by 10.5+1.35 = 11.85