Draw it out. The cross-section in the yz-plane is half of a parabola: the heights of the triangles.The easiest way to approach the volume of this shape is to use your knowledge of the area of each triangle.
This is a standard homework problem so I am moving this to the homework section.
It will help you to know that if an equilateral triangle has side length s, then it has altitude [itex]\sqrt{3}s[/itex] and so area [itex](\sqrt{3}/2)s^2[/itex].