Explanation: \(\frac{1}{2}\times a\times7+\frac{1}{2}\times a\times3+\frac{1}{2}\times a\times5=\frac{\sqrt{3}}{4}a^{2}\) \(\Rightarrow15a=\frac{\sqrt{3}}{2}a^{2}\) \(a=10\sqrt{3}\) \(Area=\frac{\sqrt{3}}{4}\times(10\sqrt{3})^{2}\) \(=75\sqrt{3}\) (Ans. A)