The expression H(t) = 70 + 100(0.819)t makes sense only for non-negative values of t
domain: t ≥ 0
How to find the domain
The domain for the situation represent the valid value for the variable t in the context of the problem.
In this case, time t should be a non-negative quantity since you cannot have a negative time in the context of this problem.
Therefore the appropriate domain for the situation is
t ≥ 0