The lateral view of the frustrum

\(\tan \theta = \frac{6}{1.5} = 4\)
\(\theta = \tan^{-1} 4 = 75.96°\)
Extending to the full height of the pyramid, we have

The height of the pyramid which formed the frustrum = (6 + x)m,
\(\tan 75.96 = \frac{6 + x}{2.5}\)
\(6 + x = 2.5 \times 4 = 10.0\)
\(\therefore \text{The height of the pyramid} = 10.0m\)