Explanation:
The formula i.e.
Fuel Efficiency = 32-0.1*speed
represents that the efficiency on very lower speeds will be affected very minor as the multiple of 0.1 which will be subtracted from 32 will be small value.
Secondly, as per attached full question and graph plot, it is clear that up to 26.5 predicted (mpg), the trend of Residual (mpg) is increasing while after 26.5, it starts to decrease nearly linearly.
It's slop can be find out by two point formula i.e.
slope = m =(y2-y1)/(x2-x1)
=(1-2.5)/(27.5-27)
=(-1.5)/(0.5)
=-3
negative sign shows decreasing trend of second half plot
