Answer
$c=22.5$
Work Step by Step
$a^{2}+b^{2}=c^{2}$
$13.5^{2}+18^{2}=c^{2}$
The number is 18 as the roof has two sides and we are simply trying to find one side. Finding one side forms a right angled triangle which we can use trig on, this splits the original 36 in half.
$182.25+324=c^{2}$
$506.25=c^{2}$
$c=22.5$
