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We know that perpendicular lines have slopes which are negative inverses of each other. This means that the slope of the line we are going to need to find is 3.
Now, we can use the point-slope formula to find the actual equation of the line we are trying to find. The point-slope formula is
, where 
and 
are the coordinates of a point on the line and m is the slope.
Thus, the equation of the line we are trying to find is:
(y - 7) = 3(x - 2)
y - 7 = 3x - 6
y = 3x + 1
However, this is not an answer choice. The answers given have the constant on the right side of the equation. Thus, we will make our answer have the constant on the right side:
y - 3x = 1
-3x + y = 1
3x - y = -1
This is an answer choice. B, or 3x - y = -1 is the correct answer.
Now, we can use the point-slope formula to find the actual equation of the line we are trying to find. The point-slope formula is
Thus, the equation of the line we are trying to find is:
(y - 7) = 3(x - 2)
y - 7 = 3x - 6
y = 3x + 1
However, this is not an answer choice. The answers given have the constant on the right side of the equation. Thus, we will make our answer have the constant on the right side:
y - 3x = 1
-3x + y = 1
3x - y = -1
This is an answer choice. B, or 3x - y = -1 is the correct answer.
Given:
Book club A charges $20 for membership and $2 per book rental.
Book club B charges $10 for membership and $3 per book rental.
To find:
The number of books for which the cost be the same at both book clubs.
Solution:
Let x be the number of books.
Book club A charges $20 for membership and $2 per book rental. So, the total cost is
...(i)
Book club B charges $10 for membership and $3 per book rental. So, the total cost is
...(ii)
Equating (i) and (ii), we get
The total cost is
Therefore, the total cost at both book clubs are same for 10 rental books and that cost is $40.