Hi there!
"MPG" is an abbreviation for "mile per gallon".
In order to answer your question you'll need to use the cross product method:
856 miles → 29.4 gallons
x miles → 1 gallon
(856 × 1) ÷ 29.4 = x miles
856 ÷ 29.4 = 29.1156463
Since you need to round your answer to the nearest, your answer will be: Eli drove 29 miles per gallon.
There you go! I really hope this helped, if there's anything just let me know! :)