Looking at the graph we can see that when the average temperature is 50º F, the grass growth is around 15 cm, when the temperature is 60º F it's 13 cm.
The best prediction for 55º F will be the point between 50º F and 60º F, as we can see, 55º F is the middle point between 50 and 60ºF.
Using that logic, we can say that the grass growth will be the middle point between 15 cm and 13 cm, which lead us to the answer: 14 centimeters.
We can also see it graphically
