You have to find two definite points on the graph, i'll use (1,4) and (2,6) as an example
The slope formula is (y2-y1)/(x2-x1)
This means you have to subtract the first y value from the second divide it by the first x value subtracted from the second
In my example you have to do (6-4)/(2-1) to get 2/1, or 2
Hope this helps!
