In the year 830, Mohamed Ben Musa Alkarismí, one of the most notable sages of the 9th century, subtracted whole numbers as follows:
(So you can keep track of operations, we'll use modern digits here.)
From 12025 let's take 3604.
The operation was started from the left (operation I). So the 12 took 3 and 9 left; canceled the digits considered (12 and 3) and wrote the rest obtained on top of the "minuendo".
Continuing: at 90 it took 6 left 84. The difference obtained (operation II) was written about the "minuendo" and the digits that formed the subtraction terms were canceled.
Finally, 8425 took 4 and 8421 remained (operation III).
And so we have the difference between the given numbers.