How is this question worked out? Watch
You can ignore 11 because the digit '1' repeats itself and we are told the code must have all digits being different.
For every option of the first number, you multiply it by 3 to obtain the second number. If any digits repeat between these two numbers, you can ignore this whole row and move onto the next option for the first number. Hence why numbers are circled in red, to indicate a repeating digit. So those rows are no good for us.
If the second number looks good, multiply the first number by 6 to obtain the third number, looking out for any repeating digits once again.