You will subtract both numbers in that order that it is written. If it was put as 3.9 and 8.3, then subtract the 2 numbers.
Since 3.9 is less then 8.3, this will be a little tricky. I will show you a hack I use.
Since we know 3.9 is less then 8.3, that means the answer will be a negative answer.
I subtract 8.3-3.9 first, which is 4.4.
Since it is 3.9 that is first, 4.4 will change to negative 4.4.
-4.4 will be your answer.
Hope this helped, and please mark brainliest! :)