Answer: 
Step-by-step explanation:
Given: Perimeter of triangular garden = 
The sum of two sides of the garden = 
Perimeter is the sum of all sides of a triangle.
Third side = Perimeter of triangular garden - sum of two sides of the garden

The expression represent the third side =
.
1. $1.23 per pair
A dozen is 12 so you do 12x7=84
Then you do 103.32 divided by 84 which gives you 1.23
2. His error was doing 103.32 divided by 7 because he thought it was only 7 pairs but it’s 7 dozen.
The answer would be multiplication and subtraction