We need to find the distance, in miles, that O'Neill ran.
We know that Robbie ran 21 miles less than O'Neill. This is the same as saying that O'Neill ran 21 miles more than Robbie.
Also, we know that Robbie ran 17 miles.
Then, to find the distance O'Neill ran, we need to add 21 miles to the 17 miles Robbie ran.
We obtain:
[tex]17\text{miles}+21\text{miles}=38\text{miles}[/tex]