Step-by-step explanation:
This is how you would proceed:
Centered at D, draw an arc of 7m (3.5cm) within the garden (rectangle).
Centered at B, draw an arc of 4m (2cm) within the garden.
There will be two intersection points, one is outside the garden (that you will not see), and the other is within the garden, which is where the tree should be planted.
See illustration attached.