we know that
The height decreases in the interval {3,4} and {4,6}
step 1
Find the slope at the interval {3,4}
m=(20-80)/(4-3)
m=-60 -----> is negative because is decreasing
step 2
Find the slope at the interval {4,6}
m=(0-20)/(6-4)
m=--20/2
m=-10
therefore
at the interval {3,4) the water ballon’s height decreases the fastest