[tex]\begin{gathered} \text{The triangle C is equilateral, } \\ So\Rightarrow x+x+x=42 \\ \Rightarrow3x=42 \\ \Rightarrow x=14 \\ x\text{ is the side of the triangle C,} \\ \text{Now, for triangle D, } \\ \text{The side is y for triangle D} \\ So,\Rightarrow y=\frac{x}{2}=\frac{14}{2}=7 \\ \text{The perimeter of D is,} \\ \Rightarrow y+y+y=7+7+7=21 \end{gathered}[/tex]
