The atmospheric pressure \(P\) in pounds per square inch (psi) is approximated by

\[P=14.7 e^{-0.21 a}\]

where \(a\) is the altitude above sea level in miles. If the atmospheric pressure of Denver is 11.9 psi, estimate Denver's altitude. (Recall that \(1 \mathrm{mi}=5,280 \mathrm{ft}\).)