Answer
The heat loss $L$ is $1800\ \text{Btu per hour}$.
Work Step by Step
According to the question$L\propto A\cdot D$ which can be written as $L=kAD$ where $L$ is loses, $A$ is the area, $D$ is temperature difference and $k$ is constant.
Substitute $A=3.6$ , $L=1200$ and $D=20$ in the above formula to calculate the value of k.
$\begin{align}
& L=kAD \\
& 1200=k\left( 3\cdot 6 \right)\left( 20 \right) \\
& k=\frac{1200}{360} \\
& k=\frac{10}{3}
\end{align}$
Thus,
$k\approx 3.33$
Now the equation becomes,
$\begin{align}
& L=kAD \\
& L=3.33\left( 9\cdot 6 \right)\left( 10 \right)\ \ \ \ \ \left( \because \ A=54\ \And D=10 \right) \\
& L=3.33\left( 54 \right)\left( 10 \right) \\
& L=3.33\left( 540 \right)
\end{align}$
So,
$L\approx 1800\ \text{Btu per hour}$
Therefore, heat loss L is $1800\ \text{Btu per hour}$.
