Question 283634
you need to calculate the three dimensional diagonals. top right to bottom left.
I did this awhile back. d=sqrt(l^2+h^2+w^2)=sqrt(4^2+3^2+2^2)=sqrt(29)=about 5.38 ft

http://www.algebralab.org/lessons/lesson.aspx?file=Geometry_3Dprisms.xml