The expression for this scenario is:
[tex]\begin{gathered} E=3000-500t \\ E\text{ is the elevation } \\ t\text{ is the time} \\ \text{From the question, t = 3 hours} \end{gathered}[/tex]Now substitute the value of t into the elevation expression above, to get the elevation reached after 3 hours.
[tex]\begin{gathered} E=3000-500t \\ E=3000-500(3) \\ E=3000-1500 \\ E=1500\text{ feet.} \\ \text{The elevation after 3 hours is 1,500 feet.} \end{gathered}[/tex]