Addition in base 12 (A=10; B=11)
A734
+ 128B
----
In base 10, the addition in the rightmost column is 4+11 = 15; in base 12 that is "13". The 3 is the first digit in the sum; the 1 gets "carried":
1
A734
+ 128B
----
3
In base 10, the addition in the next column is 1+3+8 = 12; in base 12 that is "10". The 0 is the next digit in the sum; the 1 gets carried:
11
A734
+ 128B
----
03
In base 10, the addition in the next column is 1+7+2 = 10; in base 12 that is A. The next digit in the sum is A and there is no carry:
11
A734
+ 128B
----
A03
In the last column, the addition in base 10 is 10+1 = 11; in base 12 that is B. The last digit in the sum is B and we are done.
11
A734
+ 128B
----
BA03