Answer:
3.6 miles
Step-by-step explanation:
Set up an equation. Start with the total goal of 21.4 on one side, then think about what goes on the other. He's already ridden 7 miles, and this is going to be the same as the 21.4, so start with a 7. Then, in addition to those 7, he'll need to ride some amount on average for 4 more days, so that would be + 4x. This equation its 21.4 = 7 + 4x, and now just solve for x.
21.4 = 7 + 4x
14.4 = 4x
3.6 = x
Don't forget your units! (: Also, let me know if any part of this needs a more in-depth explanation.