maths you need to answer

not a math person:

what is the third significant number of 21.3628300444

the first significant figure is 2
the second significant figure is 1
the third significant figure is 3

If you wanted to round it of to 3 significant figures, you would use the 4th one for rounding.
so if the 4th one is 5 or bigger, the 3 rd on goes up.
So
correct to 3 significant figures this is 21.4