Which value on the number line is the best estimate of the center of the data set?

a line plot with twenty-two data values. labels are at zero, three, six, nine, and twelve. tick marks are every one unit. values appear as x marks above the line. plot data values are one x mark above one, one x mark above two, two x marks above three, two x marks above four, three x marks above five, four x marks above six, three x marks above seven, three x marks above eight, two x marks above nine, and one x mark above eleven.