AABC~ AQRS
Find the missing side length, m.

Ratio remains same
[tex]\\ \rm\bullet\rightarrowtail \dfrac{AB}{AC}=\dfrac{RQ}{QS}[/tex]
[tex]\\ \rm\bullet\rightarrowtail \dfrac{3}{1}=\dfrac{m}{5}[/tex]
[tex]\\ \rm\bullet\rightarrowtail m=15[/tex]