For this question, the 2D triangle is the cross-section of this prism, since it can be multiplied by a number to become the 3D prism in the question. To understand it better, imagine having triangular pieces of papers with the same dimensions, and stacking them all up. You will end up with a 3D prism.
What you have to do next, is identify the method to solve this problem: Volume of prism = Cross section × length of prism = Area of triangle × 8 = 0.5 × (5 × 7) × 8 = 0.5 × (35) × 8 = 0.5 × 35 × 8 = 0.5 × 8 × 35 = 4 × 35 = 140 units^3 {Since the unit of the lengths of prism are not specified, use the word units instead. Also, since the value 140 is the volume of an object, remember to put the power of the unit to 3}
