I can't figure this question out I've tried multiple times but only 2/3. Help, please.

An angle that measures between 90 and 180° is called a(,

The fence around a quadrilateral-shaped pasture is 3a^2+15a+9 long. three sides of the fence have the following lengths: 5a, 10a-2, a^2-7. what is the length of the fourth side of the fence?

How is this equation completed? i cannot find any examples in the book.