a. The first variable is x and the second variable is y.
b. The equations are
and 
Step-by-step explanation:
Step 1:
The first step is to define the variables. The variables can be any two symbols, letters, characters, etc.
Here let the first variable be x and the let the second variable be y.
So the variables are defined as x and y.
Step 2:
The sum of the given variables is 12.
The first variable + the second variable = 12,

The difference between the two variables is 4.
The first variable - the second variable = 4.

Step 3:
If we add both the equations we get,
and 
x = 8 and y = 4.
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Formula
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a² + b² = c²
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Find Diagonal
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9² + 7² = c²
c² = 81 + 49
c² = 130
c = 11 in (Nearest whole number)
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Answer: The diagonal measure 11 inches
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The graph shows a horizontal line intersecting (0, 2).
Your answer is: y = 2.