so, the bottom-left angle is 60°, and the bottom-right angle is 60° as well, that simply means the angle up above is also 60°, meaning, this is an equilateral triangle.
[tex]\bf \textit{height of an equilateral triangle}\\\\ h=\cfrac{s\sqrt{3}}{2}~~ \begin{cases} s=side's\\ \qquad length\\[-0.5em] \hrulefill\\ h=4\sqrt{15} \end{cases}\implies 4\sqrt{15}=\cfrac{s\sqrt{3}}{2}\implies 8\sqrt{15}=s\sqrt{3} \\\\\\ \cfrac{8\sqrt{15}}{\sqrt{3}}=s\implies \cfrac{8\sqrt{3\cdot 5}}{\sqrt{3}}=s\implies \cfrac{8\underline{\sqrt{3}}\cdot \sqrt{5}}{\underline{\sqrt{3}}}=s\implies 8\sqrt{5}=s[/tex]
