Answer:
+7
Step-by-step explanation:
if you go from 28 to 21 you see there is a difference of 7 and sense the number is a smaller number now we know the dolphin went up more towards the surface there for there is a change of positive 7
28-21=7
9,10,11,12,13,14,15,16,17,18,19 and 29
Hope this help!!
There is no picture below?