The formula for the volume of a triangular pyramid is:
V=1/3AH (where A=area of the base and H=height)
The formula for the volume of a triangular prism is:
V=AH (where A=area of the base and H=Height)
Since the base and the height are the same in this problem for both the prism and the pyramid, solving for the volume of the prism is simple.
Looking at the formulas, you'll see that the volume of the pyramid (assuming that both the height and the area of the base are the same) is 1/3 the volume of the prism.
V(pyramid)=1/3V(prism)
Now lets input the volume of the pyramid
26cm³=1/3 V(prism)
Divide both sides by 1/3
78cm³=V(prism)
Answer=78cm³