You have some data points labeled by [tex]x[/tex]. They form the set {3, 5, 7}.
The mean, [tex]\bar x[/tex], is the average of these values:
[tex]\bar x = \dfrac{3+5+7}3 = \dfrac{15}3 = 5[/tex]
Then in the column labeled [tex]x-\bar x[/tex], what you're doing is computing the difference between each data point [tex]x[/tex] and the mean [tex]\bar x[/tex]:
[tex]x=3 \implies x-\bar x = 3 - 5 = -2[/tex]
[tex]x=5 \implies x-\bar x = 5-5 = 0[/tex]
[tex]x=7 \implies x-\bar x = 7 - 5 = 2[/tex]
These are sometimes called "residuals".
In the next column, you square these values:
[tex]x=3 \implies (x-\bar x)^2 = (-2)^2 = 4[/tex]
[tex]x=5 \implies (x-\bar x)^2 = 0^2 = 0[/tex]
[tex]x=7 \implies (x-\bar x)^2 = 2^2 = 4[/tex]
and the variance of the data is the sum of these so-called "squared residuals".
