The rat evaporates 1 ml of water per hour, that would remove about 0.6 watts of heat energy (starting at 311 K, the kangaroo rat's body temperature). From your calculations on the right, you now know how much heat energy the k-rat needs to lose during the middle of the day in the desert to stay in homeostasis.
If the kangaroo rat were to lose that heat by water loss (the only mechanism left in the heat balance equation), how much water (in ml) would it need to lose every hour?