The annual discount is 3%, therefore the total discount for 1 year would be:
4300 × 3 ÷ 100 = 129$
Now, you can set a proportion:
discount : annual discount = loan days : days in one year
21 : 129 = x : 360
x = 21 × 360 / 129 = 58.6 days
Hence, the loan duration was 60 days.