Question 244115
You are correct.
You didn't state the whole problem.
I will assume you want to know how long Ethan swam. There are other possible questions. 
N=time Ethan swam
L=85
85=21+4n
and your solution comes in next
Subtract 21 from both sides

85-21=4n
simplify

64=4n
divide both sides by 4
16=n
Ethan swam 16 minutes.