7.4 is the mean of the data.
The mean is the same thing as the average. To find the mean, you add all of the values up (so in this case, 2+2+3+4+4+4+4+5+5+6+6+7+7+8+8=111) then you divide that number by the number of numbers (I hope that made sense). Since there are 15 numbers here, we divide by 15. 111 divided by 15 = 7.4.