The expression \( (f + 9)(t) \) represents the height of the rocket that is 9 feet higher than its original height \( f(t) \). This could describe a situation where the rocket is launched from a platform that is 9 feet above the ground level, thereby adding 9 feet to the height of the rocket at any given time \( t \).
The domain of \( (f + g)(t) \) would depend on the height function \( f(t) \) and the constant function \( g(t) \); in this case, it would be defined for the time intervals where the height of the rocket is meaningful (i.e., when the rocket is in flight and has not yet returned to the ground). Specifically, it would encompass the time from launch until the rocket reaches the ground again, indicating that \( t \) would be a non-negative interval starting from 0 up to the time of landing.