Cell Format will not take

I have entered a formula: =IMSUB(C5,B5) C5 is 300.8 and B5 is 298.2, and I want a simple
-2.6 answer, but no matter how I format it, it comes out -2.60000000000002.
I have made every change I could under General and Number cell formatting to no avail.
I went in to Excel options to make some changes, unclicking “Use system separators” and designating the Decimal separator to 1. But that did not work, so put it back like it was.
I have been everywhere! to find help and spent 4 hours reading to no avail. Can someone please help me?