we know that
[volume of triangular prism]=[area of the base]*h
[volume of a triangular pyramid]=[area of the base]*h/3
then
[volume of triangular prism]/[volume of a triangular pyramid]=3
hence
[volume of a triangular pyramid]=[volume of triangular prism]/3
the answer is
[volume of a triangular pyramid]=(1/3)*[volume of triangular prism]
see the attached figure