Yaks need food and water, and use more than goats. Farmer Bacon knows how much food and water his goats consume (relative to a Yak) - a full circle round a goat is the requirement for a WHOLE Yak. So you need to select (by clicking) goats to combine together to replace with a Yak.
Be warned if the Yak has insufficient food and water it will die! So how many viable Yaks can you replace Farmer Bacon's goats with?