Question 788226
f for part one, as in First;
s for part two, as in Second.


{{{f+s=21}}} because the two parts together are the 21 mile trip.


{{{f=s-9}}}, according to the description's latter part.  


The rest of the solution is for you to handle.  The more difficult work is to analyze and obtain those two equations.  (A simple substitution should be obvious).