OK, let's do this. We've got 99121 and 40590, and we're subtracting the second number from the first.

Step 1: We'll start by subtracting the digit 0 and the borrow 0 from 1 in column 1 and get 1.
1 is the first digit of our result.

Step 2: We'll start by subtracting the digit 9 and the borrow 0 from 2 in column 2 and get -7.
We add -7 to 10 and get 3 as the next digit of the result.

Step 3: We'll start by subtracting the digit 5 and the borrow 1 from 1 in column 3 and get -5.
We add -5 to 10 and get 5 as the next digit of the result.

Step 4: We'll start by subtracting the digit 0 and the borrow 1 from 9 in column 4 and get 8.
8 is the next digit of our result.

Step 5: We'll start by subtracting the digit 4 and the borrow 0 from 9 in column 5 and get 5.
5 is the next digit of our result.

So, 99121 - 40590 = 58531.