The function \( E(t) = 3600 - 90t \) describes Terry's elevation over time, with an initial elevation of 3600 feet and a descent rate of 90 feet per second. Specific elevations at given times are:
- At \( t = 0 \) seconds, \( E(0) = 3600 \) feet.
- At \( t = 10 \) seconds, \( E(10) = 2700 \) feet.
- At \( t = 20 \) seconds, \( E(20) = 1800 \) feet.
\[
\boxed{E(0) = 3600 \text{ feet}, \quad E(10) = 2700 \text{ feet}, \quad E(20) = 1800 \text{ feet}}
\]