The numerical series for geniuses

We present a numerical series that they count is only suitable for geniuses. The solution is not very complicated, but it forces us to think differently than we are used to.

Look at this series of operations. Apparently, they all have an incorrect result, but they follow a pattern that you must guess to Find the missing numbers in the last operation.

Take your time and don't forget to keep your mind open. Anyway, you know that you have the hidden solution a little below.

Luck!

The result of each operation is calculated as the subtraction of the two numbers of the concatenated operation with the sum of the same numbers.

In the last operation, we must find two numbers x and y that meet the following:
X - y = b
X + y = a
Ba = 123

If you solve the system, we get that 12 + 11 = 123.