The formulas for finding the slope and the y-intercept of the line of best fit use both

Question:

The formulas for finding the slope and the y-intercept of the line of best fit use both summations ’s, and sums of squares, SS( )’s. It is important to know the difference. In reference to Example
3.5 (p. 138):
a. Find three pairs of values: _x2, SS(x); _y2, SS(y); and _xy, SS(xy).
b. Explain the difference between the numbers for each pair of numbers.