Thank you for the opportunity to help you with your question!
What you need to do is make the 6 into a decimal as well. 6 is a whole number so it is simply followed by 0's.
What you are going to do is 5.99-6.00 and just proceed with normal subtraction. Switch the order of the subtraction to 6.00-5.99 and just remember to change the sign of your answer because the order of subtraction is in reverse.
6.00 - 5.99 = 0.01 Just use the borrowing method you would do in normal subtraction and just bring the decimal point down as show above. Now change the sign because you are actually doing the subtraction in reverse. The answer is... -0.01 Please let me know if you need any clarification. I'm always happy to answer your questions.