The equilateral triangle may be splited into two congruent isosceles right triangles, each with height 15, and angles 30°-60°-90°.
Then you have this ratios:
tan (60) = height / [base of the equilateral triangle / 2] => base = 2*15 / tan(60) = 30/√3
sin (60) = height / hypothenuse => hypothenuse = height / sin(60) = 15 / [√3 / 2] = 30 / √3.
Same result of above which is right because the three sides are equals.
Then, side of the equilateral triangle is 30 / √3
