A median.
The "middle" of a sorted list of numbers. To find the Median, place the numbers in value order and find the middle number.
When there are two middle numbers we average them.
We have:
3.9, 7, 3.3, 1, 1.4, 1.7, 2.1, 3.3, 5.2
Let's sort the numbers from the smallest to the largest
1, 1.4, 1.7, 2.1, 3.3, 3.3, 3.9, 5.2, 7
The median is 3.3