Distance = rate * time
------
t = faster time
2 - t = slower time
8(2-t) + 12t = 21
======Your equation. Simple to solve, just don't forget 2 - t to get slower time.
At a high-school cross-country meet, Jared jogged 8mph for the first part of the race, then increased his speed to 12mph for the second part. If the race was 21 mi long and Jared finished in 2 hr, how far did he jog at the faster pace?