Answer
$ 138,674.2$ feet
Work Step by Step
Given \begin{equation}
T(h)=0.243 \sqrt{h}.\tag{1}
\end{equation} Set $T(h)=1.5\cdot 60 = 90$ seconds and solve equation $(1)$ for $h$.
\begin{equation}
\begin{aligned}
T(h)& = 90\\
0.243 \sqrt{h}&=90\\
\sqrt{h}&= \frac{90}{0.243}\\
\left( \sqrt{h} \right)^2&= \left( \frac{90}{0.243} \right)^{2}\\
h& = 137,174.2.
\end{aligned}
\end{equation} The plane should be at a height of $H=137,174.2+1500= 138,674.2$ feet to allow for the daredevil to pull is ripcord $1.5$ minutes at $1500$ feet.
