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.
∆ABC=∆DEF
AB=DE –>String
BC=EF–> Rib
m<C=m<F=90° –>List
AC=DF
So m<A=m<D=35 (It is not clear whether the number is 35 or 36, but the same number)
Every degree is a touching point since each root factor is an x intercept
therefor the answer is 8th degree