Base Twelve.

B08twelve + ? = 9twelve

So is your question using base twelve, 08+?=9

or is it
Base eight, 12 + ?= what in base 12?

I have no idea what your question is

Are you supposed to work your arithmetic within the base 12 ?

If so, then you should have an addition table, which you can use also for subtraction

Other option:
switch the numbers to base 10, do the arithmetic, then switch back.
B08twelve + ? = 9twelve
(11(12^2) + 0(12) + 8) + ? = 9
1592 + ? = 9
the missing number is -1583
which in base 12 = -AB1

to work within the base
B08
- 9 I will have to borrow, all the way from the B
-----
AB1

To solve this problem, we need to convert the given numbers from base twelve to base ten.

In base twelve, the digits range from 0 to 9 and then use letters A and B to represent 10 and 11. Therefore, B08twelve can be converted to Base Ten as follows:

B08twelve = (11 * 12^3) + (0 * 12^2) + (8 * 12^1) + (11 * 12^0)
= (11 * 1728) + (0 * 144) + (8 * 12) + (11 * 1)
= 19152 + 0 + 96 + 11
= 19259

Now let's find the value of the missing number:

9twelve = (9 * 12^1) + (9 * 12^0)
= (9 * 12) + (9 * 1)
= 108 + 9
= 117

To find the missing number, we subtract the value of B08twelve from 9twelve:

117 - 19259 = -19142

Therefore, the missing number, when added to B08twelve, is -19142.