To determine the equation of the line in this case, you need to choose two points of the graph.
Let
Point 1
P1 = (x1, y1)
Point 2
P2 = (x2, y2)
The slope of the line is
m = (y2-y1) / (x2-x1)
The equation of the line (choosing any point) will be
y-y1 = m (x-x1)
For this particular case, the line that best represents the graph is:
y = -0.35x + 5.3