The rates of change can be computed using the slope formula.
m = (y2 -y1)/(x2 -x1)
The points used and the resulting "rate of change" are shown in the attachment.
Table C shows a rate of change (1/3) that is less than that of the graph (1/2).
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I like to use a spreadsheet for repetitive calculations, especially ones involving negative numbers. This helps prevent errors (and tedium). The formula is entered only once.