Let the length of equilateral triangle be x.
Each of the angle in an equilateral triangle is = 60°
The opposite to the 60° is the 18 inches. Hypotenuse is the side.
Sin 60° = 18 / Hypotenuse
Hypotenuse = 18 / Sin60°. Sin60° = √3 / 2
x = 18 / (√3 / 2)
= 18 / √3 * 2/1 = 36 / √3
x = 36 / √3
Rationalize the surd = 36 / √3 * (√3 / √3 ) = 36√3 / 3
x = 12√3 ≈ 12 * 1.732 ≈ 20.784
Length of side, x = 12√3 ≈ 20.784 inches.