Check the picture below, the assumption being that the 6ft line is perpendicular to the 18ft line and thus making a right-triangle.
now, if we just get the whole area of the containing parallelogram and then get the area of the triangle and subtract it from that of the parallelogram's, what's leftover is the area we didn't subtract, the shadead area.
[tex]\stackrel{ \textit{\LARGE Areas}}{\stackrel{parallelogram}{(18)(10)}~~ - ~~\stackrel{triangle}{\cfrac{1}{2}(18)(6)}}\implies 180~~ - ~~54\implies 126[/tex]
