Explanation:
the slope of a line is its inclination and is expressed as ratio of "y coordinate change / x coordinate change" when going from one point to another on the line.
now, I see 2 "good" points on the chart (I mean by that points with integer coordinates) :
(0, 10)
(20, 100)
so, between these 2 points
x changes by +20 (from 0 to 20).
y changes by +90 (from 10 to 100).
so, the slope is +90/+20 = 9/2