recall your d = rt, distance = rate * time
the distance the plane travelled over and back was the same distance, say "d", since it simply went over to that city and then it came back from it.
if it took say "t" hours to come back, on the way over, it took then " t + 1 ", because it took longer on the way out by 1 hour.
and we know the speed rates.... ok
[tex]\bf \begin{array}{lccclll}
&distance&rate&time\\
&-----&-----&-----\\
\textit{going out}&d&300&t+1\\
\textit{coming back}&d&350&t
\end{array}
\\\\\\
\begin{cases}
\boxed{d}=300(t+1)\\
d=350t\\
----------\\
\boxed{300(t+1)}=350t
\end{cases}[/tex]
solve for "t" to see how many hours it took on the way back.
the whole trip took then (t + 1) + t