Sum of interior angles of a hendecagon ##=1620^@##
The real issue here is knowing that a hendecagon is an ##11## sided (and angled) polygon.
The formula for the sum of the interior angles of an ##n##-gon is
##color(white)(XXX)180^@xx(n-2)color(white)(XXXXX)##provided ##n>=3##
An ##11##-gon (hendecagon) can be divided into ##9## triangles by connecting vertices; each triangle has an interior angle sum of ##180^@##
Sum of interior angles of hendecagon ##=180^@ xx 9 = 1620^@##