This question is a problem under "Enlargement and Similarities"
Triangle ABC is enlarged to form triangle ADE.
This means AB is enlarged to form AD and CB is enlarged to form ED. So I'm order to find x we compare the ratio of AD and AB to CB and ED.
AB/AD =CB/ED
9/(9+6) = 3/x
=9/15 = 3/x
Multiplying through by 15x
= 9x =45
= x= 45/9
=x=5
