Ok ok so I saw that someone else had given you an answer but I thought it might be best if you got an explanation to go with it.
Basically to know what your final price is you need to know what percentages represent.
Since they are simply parts of a whole (because you know 100% of something is a whole) we can look them as fractions like 40/100 in this case.
Then we can multiply 39.00 by 40/100 or 0.40 in decimal form and get 15.60.
It’’s important to remember that this 15.60 is just what 40%of 39 is so we still need to subtract it from our total to get the sales price.
39-15.60=23.40
So $23.40 is the sales price
Hope this helps!