I can't be entirely sure what you want to "do" about the slope,
but here is how to FIND it if you have the line on the graph:
-- You pick two points on the graph line.
-- Find the difference in 'y' between the two points.
-- Find the difference in 'x' between the two points.
-- The slope of the line between the two points is
(the difference in 'y')
divided by
(the difference in 'x') .
-- If the line on the graph is a straight line, then
the slope is the same everywhere on it.