[tex]\begin{gathered} \text{Perimeter =6a+3}.\text{ Let L be the lenght of side number 3. Then,} \\ 2(a+3)+3a-1+L=6a+3 \\ \text{Hence,} \\ 2a+6+3a-1+L=6a+3 \\ 2a+3a+6-1+L=6a+3 \\ 5a+5+L=6a+3 \\ L=6a+3-5a-5 \\ L=6a-5a+3-5 \\ L=a-2 \end{gathered}[/tex]
