"The turtle's legs are long but slow." <-- this was marked wrong.
the turtles legs are long but it (the turtle not the legs) is slow
But how do you know which noun the final verb is referring to?
In that case, these are not verbs, they are adjectives. They refer to the noun that has the 이/가 markers. Here, the "turtle's legs".
It didn't accept "it's" instead of "it is." Seriously...