I use strange methods when undertaking mental arithmetic (which this is - no calculators are necessary). I deducted 5364 from the nearest thousand (6000) and got 636. (I even arrive at this by a convoluted method, saying that 36 is needed to round up 364 to 400 and 400 is 600 less that 1000, so the answer 36 + 600). Then I took 6000 from 8848 (easy, 2848). Then I added the 636 to 2848 to get to 3484.
At school we were encouraged to find the easiest way (for each of us individually) to do these sums. I've always found this method the easiest in most cases but I use all sorts of convoluted methods. Took me about ten seconds - far less than it would have done to find my calculator (and probably find a new battery to go in it because it is seldom used). Not everybody's favourite methodology but it works for me.
By the way, buzz, there was a "homework" question on here a couple of weeks ago where the information given in the question was clearly incorrect:
https://www.theanswerbank.co.uk/How-it-Works/Question1577929.html
So it's unlikely you are missing anything but more likely that shoddy teaching is to blame.