First of all, we need to remember thatif you have two rectangles that are similar, it means that thet have a relation of proportion between its sides... We are gonna make a draw:
I drew in blue the rectangle ABCD and in green the rectangle EFGH. Note that I change the ubication of the segments to make it proportional, but it doesn't change the answer.
Now, I make a relation:
[tex]\begin{gathered} \frac{10.5}{3.5}=\frac{x}{5} \\ 3=\frac{x}{5} \\ x=15 \end{gathered}[/tex]Finally, the answer is 15.