Explanation:
the height of the prism is the missing side of the triangular "side".
Pythagoras
c² = a² + b²
with c being the Hypotenuse (the side opposite of the 90 degree angle).
10² = 6² + height²
height² = 10² - 6² = 100 - 36 = 64
height = 8 in
the volume of the prism is now
length × width × height / 2 (due to the triangle "cut" only half of the full "box" remains for this prism).
so we have
14 × 6 × 8 / 2 = 14 × 6 × 4 = 336 in³