Proposition: Prop. 11.39: Prisms of Equal Height with Parallelogram and Triangle as Base

(Proposition 39 from Book 11 of Euclid's “Elements”)

If there are two equal height prisms, and one has a parallelogram, and the other a triangle, (as a) base, and the parallelogram is double the triangle, then the prisms will be equal.

Let $ABCDEF$ and $GHKLMN$ be two equal height prisms, and let the former have the parallelogram $AF$, and the latter the triangle $GHK$, as a base.
And let parallelogram $AF$ be twice triangle $GHK$.
I say that prism $ABCDEF$ is equal to prism $GHKLMN$.