Ok, in a Venn Diagram each circle represents a certain characteristic. In this case we have two circle since we have two characteristics: brown eyes and blond hair. Now, deppending of the characteristics we are talking about is possible that an individual has many of them at the same time. Here it's possible to have both blonde hair and brown eyes at the same time. The area that is shared by both circles represents this group of children.
All in all, the areas of the circles that are not shared by both represents the children with blonde hair only and the children with brown eyes only. Therefore one of this areas will have a 15 and the other needs to have a 19. For the common area shared by both circles you need to fill it with number 9.