If sides are
[tex]\\ \sf\longmapsto Perimeter=Sum\:of\:sides[/tex]
[tex]\\ \sf\longmapsto Perimeter=9+4.5+12[/tex]
[tex]\\ \sf\longmapsto Perimeter=13.5+12[/tex]
[tex]\\ \sf\longmapsto Perimeter=25.5cm[/tex]
[tex] \large\begin{gathered} {\underline{\boxed{ \rm {\purple{Perimeter\:_{(Triangle) \: \large= \large\:Sum \: of \: all \: sides}}}}}}\end{gathered}[/tex]
[tex]\large\bf{\red{ \longrightarrow}} \tt \: \:9 \: cm \: + \: 4.5 \: cm \: + \: 12 \: cm[/tex]
[tex]\large\bf{\red{ \longrightarrow}} \tt \: \:13.5 \: cm \: + \: 12 \: cm[/tex]
[tex]\large\bf{\red{ \longrightarrow}} \tt \: \:25.5 \: cm[/tex]
Hence , the perimeter of triangle is 25.5 cm