The correct slope is -5/2.
The formula for slope is
m = (y₂-y₁)/(x₂-x₁)
The y-coordinate of the second point is 0, and the y-coordinate of the first point is 2. The x-coordinate of the second point is 0.8 and the x-coordinate of the first point is 0:
m = (0-2)/(0.8-0) = -2/0.8 = -2 ÷ 8/10 = -2 × 10/8 = -2/1 × 10/8 = -20/8 = -5/2