check the picture below.
so, notice, the prism is really just 6 rectangles stacked up to each other at the edges.
front and back, two rectangles of 11x6
left and right, two rectangles of 5x6
top and bottom, two rectangles of 11x5
get the area of all 6, and sum them up, and that's the area of the prism.
[tex]\bf \stackrel{\textit{front and back}}{2(11\cdot 6)}~~+~~\stackrel{\textit{left and right}}{2(5\cdot 6)}~~+~~\stackrel{\textit{top and bottom}}{2(11\cdot 5)}[/tex]
