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.
Step-by-step explanation:
x-intercept => point (2, 0)
y-intercept => point (0, -4)
Let that certain number be 'n'
50% (n) = 39
50% ⇒ 0.5
0.5 (n) = 39
Divide both sides by 0.5
0.5n = 39
/0.5 /0.5
n = 78.
So that certain number is actually <em><u>78! :)</u></em>
CROSS CHECK:
50% of 78
0.5 × 78
= 39
Hope this helps! :)
The correct answer is letter B