Answer:
12 feet
Step-by-step explanation:
By Pythagoras theorem, a²+b²=c², where c represents the diagonal and a and b represents the other sides of a right angled triangle
As a flag is a rectangle, the two triangles cut would be right angled triangles
Therefore, a²+9²=15²
a²+81=225
a²=144
a=√144
a=12 feet
