Use Figure 1.16 showing how the quantity, $Q,$ of grass (kg/hectare) in different parts of Namibia depended on the average annual rainfall, $r,(\mathrm{mm}),$ in two different years.
(a) For $1939,$ find the slope of the line, including units.
(b) Interpret the slope in this context.
(c) Find the equation of the line.