a) My hypothesis is in this case is that the environmental conditions in the yard were too extreme which resulted in half of the trees dying.
b) I can test this hypothesis by asking myself, what environmental conditions were the trees in that caused half of them to die? I could read about different biomes and their climates and learn how well they support organic life such as grass, plants, crops, and even trees. I can then determine that half of the trees did not survive due to the extreme environmental conditions they were in.
Hope this helps!