well, the volume of that triangular prism will just be the area of the triangle times the length.
now, what's the area of that triangular face? well, let's say it has a base of "b" and a height of 3, whilst the prism has a length of 6, Check the picture below.
[tex]\stackrel{\textit{triangle's area}}{\cfrac{1}{2}(b)(3)}\stackrel{\textit{prism's length}}{(6)}~~ = ~~4\frac{1}{2}\implies 9b=\cfrac{9}{2}\implies b=\cfrac{9}{18}\implies b=\cfrac{1}{2}[/tex]
