[tex]\begin{gathered} L\text{ = 12} \\ \text{Breadth / width = x} \\ \text{Area, a = L }\times\text{ B} \\ \text{ = 12 }\times\text{ a } \\ \text{ = 12a} \\ \text{Perimeter, P: = 2(L + B)} \\ \text{ = 2(12 + x)} \\ \text{ = 24 + 2x} \\ \end{gathered}[/tex]