The volume of the frustum of regular triangular pyramid is 135 m3. The lower base is an equilateral triangle with an edge of 9 m. The upper base is 8 m above the lower base. What is the upper base edge, in m?.

A = 9 m (edge of a lower base); b = ?; H = 8 m

V = H/3 * (B 1 + √B 1 B 2 + B 2)

135 = 8/3 (9²√3 / 4 + 9 b√3/4 + b²√3/4)

135 = 2/3 (81√3 + 9 b√3 + b²√3)

135√3 / 2 = 81 + 9 b + b²

b² + 9 b + 81 = 117

b² + 9 b - 36 = 0

b 1/2 = ( - 9 + √ (81 + 144)) / 2

b = ( - 9 + 15) / 2

b = 3 m

The upper base edge is 3m long.