If the sum of two numbers is 12 and the first number is x, the other numbers is 12 - x.
This time the expression is the result of solving an equation.
Suppose the sum of two numbers is 12.
Algebraically this is written as x + y = 12.
Solve for y:
x + y = 12
-x -x
---------------
y = 12 - x
If the sum of two numbers is 12 and the first number is x,
the other numbers is 12 - x.
You can even check this answer.
x + y = 12
x + (12 - x) = 12
x + 12 - x = 12
12 = 12