Answer
a) $W$ decreases as the $T$ increases and $W$ increases as the $R$ increases.
b) $$-1.1$$
Work Step by Step
a) (1) It has been notice that the rate of change of wheat production (W) with respect to the average temperature (T) is negative.
So, when $W_T \lt 0$, then we have $W$ decreases as the $T$ increases.
2) It has been notice that the rate of change of wheat production (W) w.r.t. the annual rainfall (R) is positive. So, when $W_R \gt 0$.
then , we have $W$ increases as the $R$ increases.
Hence, $W$ decreases as the $T$ increases and $W$ increases as the $R$ increases.
(b) We need to apply chain rule.
$$\dfrac{dW}{dt}=(\dfrac{\partial W}{\partial T})(\dfrac{dT}{ dt})+(\dfrac{\partial W}{\partial R})(\dfrac{dR}{ dt}) \\=(-2)(0.15)+(8) (-0.1)\\=-0.3-0.8 \\=-1.1$$