The sentence is: Agricolae equi in reginae terra (with a macron) sunt. The answer is "the farmer's horses are on the queen's land." What ending are you supposed to use for something following an 's or s'? Doesn't this sentence use two different endings for the same instance? Why is terra in the ablative? Isn't ablative only used for things like in, on, with or without? Then why in this instance is terra with a macron as in the ablative case? Thanks for clearing this up.

1. The farmer's horses = the horses of the farmer. The word "of" is your big clue that a possessive is coming afterward. Literally, the first part of that sentence translates as ...

The horses of the farmer...

2. terra with a macron IS in the ablative case because it's the object of the preposition in. That part of the sentence translates as...

... in the land of the queen.

in terra = "in the land"
reginae = genitive singular = "of the queen" or "the queen's"