Answer: The fence's shadow is 2.6feet long
In this case, the flagpole is 20ft height but have an 8ft length of the shadow. Using this ratio, you can count how long the fence shadow would be. Height to shadow ratio of the flagpole is:
8ft /20 ft= 0.4
Then you just need to multiply the ratio with fence height, it would be :
0.4 x 6.5ft= 2.6ft