Check out the image below to see the filled out chart.
The first column is found by plugging in the x values to get corresponding y values. For instance, if x = -5, then y is...
y = x^2+3x-18
y = (-5)^2+3(-5)-18
y = 25-15-18
y = 10-18
y = -8
So that explains how they got -8 in the second column, next to x = -5.
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The column of first differences is found by subtracting the adjacent terms in the y column. The distance from y = -8 to y = -14 is 6 units, so we'll write 6 in the first row of the "first differences" column. The rest of the values of this column are filled out in a similar fashion. This means that you'll need to completely fill out the y column before you can move onto the "first differences" column.
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The second differences is found by subtracting the adjacent first differences. You should find the results of this column are all the same value. This is true of any quadratic function.