Help! If Matching value, return 1st value-attach file.

Hi everyone,
I have limited working knowledge of excel and I hope someone will be able to help me with my problem.
What I need if to compare the digit number, if both numbers match the unique digit with containing ‘9’, ‘9’ it will return from the top 1st left number.
This formula based plus (+) and equal (=) and any random calculation number from 1 to 9 only.
For example, if we type any top ‘4’ digit number suddenly it appears matching number ‘9’ & ‘9’ like this:
.
2 – 7 – 1 - 8
‘9’ ‘9’
9
9 9
9

What I want the results become like this:
.
2 – 7 – 1 - 8
‘2‘ 9
2
2 4
6

How do I swap (change) ‘9’ to digit ‘2’ (only left digit)? Also what type of formula should I manipulate and which column should I put in the formula?
Before this, I have tried to use vlookup, match, index, and lookup at random formula but still didn’t work.
Please see my attachment.
Thank you for showing interest.
Any help is still appreciated.

Is there a Luhn algorithm aplication?

https://www.excelforum.com/excel-for...a-formula.html

In the worksheet, there are many helper columns and helpers area, I don't know what and where is 'BEFORE" and "AFTER"

Could you create new sheet which display BEFORE and AFTER with no helpers?

For example:

BEFORE: V7,X7,Z7,AB7 with 2,7,1,8 (or 2718 in cell V6,...)

AFTER: X14=9

The rest is helpers (contains steps lead to final goal in X14)

Which case (1 or 2) is your finally goal?

(In attachment, manually mock up your expectation)