The equilateral triangle can be considered to consist of two back-to-back right triangles. If the prism has a triangular base, then the base of the right prism could be one of these right triangles, so its area would be half the area (9√3cm2) of the equilateral triangle=4.5√3cm2. The volume of the prism is 15×4.5√3=67.5√3=116.91cm3 approx.
If the equilateral triangle is the entire base of the prism, then the volume would be 233.82cm3 approx.
If the prism has a rectangular base then there are many rectangles that could fit into an equilateral triangle. The question doesn't say whether it's a triangular, square or rectangular prism. A rectangular prism is also called a cuboid.