(d) Given, area of an equilateral triangle = 9√3 cm^{2}
∴ Area of an equilateral triangle = √3/4(Side)^{2}
⇒ √3/4 (Side)2 = 9√3
⇒ (Side)2 = 36
∴ Side = 6 cm [taking positive square root because side is always positive]
Hence, the length of an equilateral triangle is 6 cm.