The ratio of the golden square is
[tex]1\colon1.618[/tex]Therefore, if the shorter side is given, we multiply by 1.618 to get the longer side.
If the shorter side is given as 3.5 in, the longer side is given by
[tex]\begin{gathered} 1.618\times3.5 \\ =5.663 \end{gathered}[/tex]Therefore, the answer, to the nearest tenth, is 5.7 in.