The recipe never changes, so when prism questions go wrong it's almost always one of three things: the wrong face, mismatched units, or forgetting the answer is cubic.
Which Face Is the Cross-section?
The cross-section is the slice that stays the same shape all the way through — the face you'd see if you sliced the prism straight across like a loaf of bread. The length is how far that slice runs. If a box is lying on its side, it's tempting to grab the big face you can see and call that the cross-section, then multiply by a side that's actually part of the length. Watch the shaded end in the toy: that's the one you take the area of, and you multiply it by the dimension running away from it.
Keep Your Units the Same
You can't multiply centimetres by metres. If one measurement is in mm and another in cm, convert one so they match before you multiply. A 50 cm length and a 200 mm width? Make them both cm (20 cm) or both mm — just pick one and stick to it. Mixed units quietly wreck an otherwise perfect method.
The Answer Is in Cubic Units
The cross-section area is in square units (cm²). Multiply by a length (cm) and you've multiplied three lengths in total, so the volume lands in cubic units — cm³. Getting the number right but writing cm² instead of cm³ still drops a mark. The little ³ is telling everyone you filled a 3-D space.