That would be called the solution
Answer:
x(3+28y)
Step-by-step explanation:
21x-18x+28xy
3x+28xy
x(3+28y)
The equation for Louis's purchase is: 
The equation for Kate's purchase is: 
The equation for Biff's purchase is: 
Step-by-step explanation:
First of all we have to define variables for each item involved in the purchase
Let x represent burger
y represent soda
z be the slice of pizza
Then
"Louis bought a burger and soda for $8"

"Kate purchased a slice of pizza and soda for $9"

"Biff purchased a burger, a slice of pizza and a soda for $13.50"

The equation for Louis's purchase is: 
The equation for Kate's purchase is: 
The equation for Biff's purchase is: 
Keywords: Linear Equations, Variables
Learn more about Linear equations at:
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The last option, the y-intercept.