The mean is (8+4+16+2)/4=30/4=7.5.
The variance is ((8-7.5)^2+(4-7.5)^2+(16-7.5)^2+(2-7.5)^2)/4=115/4=28.75.
Standard deviation is √28.75=5.36.
This would give us 7.5±5.36 for the number of DVDs.
I'm not sure why 3 has been given as the sample size, but I assume that the standard deviation is to be calculated using 3 instead of 4 as the divisor. This means SD=√(115/3)=6.19 and the number of DVDs rented is 7.5±6.19 or the range 1.31 to 13.69.