[tex]\bf \textit{height of an equilateral triangle}\\\\
h=\cfrac{s\sqrt{3}}{2}\implies 2h=s\sqrt{3}\implies \cfrac{2h}{\sqrt{3}}=s\qquad
\begin{cases}
h=height\\
s=\textit{one side's length}
\end{cases}
[/tex]
anyway, the perimeter will all sides added together, namely s + s + s or 3s