Each year, the tree doubles in height, so to figure out how tall it was the previous year, we divide the height by 2.
1 year ago: 9.6m / 2 = 4.8m
2 years ago: 4.8m / 2 = 2.4m
3 years ago: 2.4m / 2 = 1.2m
The height of the tree 3 years ago was 1.2 metres.
