I'll refer to the table below as I go through the logic.
Lines 1 - 3 document what we know.
I'm curious what Aileen's commission would be if she got the full $2,000 sales amount that gives her 10%. Line 4 tells us she would get $200 if she made $2,000 in sales. She made more than that by $31.20, Line 6. The Line 6 amount would have come from sales over $2,000 at the 12% rate.
That can be calculated by the equation in Line 7. Line 8 is the result, which tells us she would have had to sell $260.00 above the $2,000 target to earned the additional $31.20. Add that to the $2,000 and we get Line 9, her total sales.
I hope this helps,