How to calculate (by hand) the slope of a line in a graph: 

(Note- In experiments using Excel functions, the slope is not to be determined using this method: it must be determined using Excel.  See Prelab 11)

Pick two points (X-value, Y-value) that are on the line.  The easiest points to pick are points where the line crosses the grid of the graph paper so you don't have to extrapolate the x and y values of the points.  In this example, Point 1 could be (5,10), i.e.,  X1 = 5 and Y1 = 10, and Point 2 could be (10,20), i.e., X2 = 10 and Y2 = 20. 

Plug these two points into the following formula to calculate the slope:

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