Problem
First name
Philippa Noble
School
King's School, Chester
Country
Age
10
first of all, you need to wait until the sun is half way up the sky so the shadow is not too stretched and it is not mid-day so it is not right above the tree.
when the time is right, you need to measure the shadow. I recommend a tape measure or a metre stick to do this. when you have the measurement you can say that this is the height of the tall tree.