The slope or gradient of a line is a number that describes both the direction and the steepness of the line. To calculate the slope of a line, you take the ratio of the change of the two points. It is expressed as follows:
slope = Δy / Δx
slope = y2 - y1 / x2 - x1
You take two points from the graph and calculate using the equation above. Hope this helps. Have a nice day.