To determine the age of a dog when a human is 3 years old based on the information provided, we can look at the relationship shown in the graph.
The line starts at the origin (0,0) and passes through the point (3,21). This means that when a human is 3 years old, the corresponding age of the dog is 21 years old.
Thus, the answer is:
A 3-year-old human is the same as a 21-year-old dog.