Variable c is 5 more than variable
a. variable c is also 3 less than variable
a. which pair of equations best models the relationship between c and a?
c = a − 5
c = a + 3
a = c + 5
a = 3c − 3
a = c − 5
a = 3c + 3
c = a + 5
c = a − 3

First, let us restate the given conditions

c is 5 more than variable a ( c = a + 5)
c is also three less than variable a (c = a - 3)

Now, lets look at the answer choices and or given
c = a − 5 
c = a + 3
Here, c is 5 less than "a"...so automatically disqualified

a = c + 5
a = 3c − 3 
Here, we have to get "C" by itself in both top and bottom equation.
So,
Simplified version :
c = a - 5
Here, c is 5 less than "a"...so automatically disqualified

a = c − 5
a = 3c + 3
Here also, we have to get "C" by itself in both top and bottom equation.
So, 
simplified version:
c = a + 5
Here, c is 5 more than "a"...so we continue
c = (a - 3) / 3
Here, c is 3 less than "a" divided by 3 . So, this is not correct

c = a + 5
c = a − 3
Here, c is 5 more than "A"
Also, c is 3 less than "a"
Which satisfies the given.

 So, our answer is going to be the last one:
 
 c = a + 5
 c = a - 3

More means to add NOT multiply
less means subtract therefore,

c= a+5
c= a-3
is the the correct choice

hope this helps