I think he meant finding the square whose area is equal to that of a given triangle.

With our "modern" understanding of algebra and geometry, we know to do

Area = 1/2 * base * height

Area = s^2

therefore

s = sqrt(1/2 * base * height)

"Squaring the circle" (https://en.wikipedia.org/wiki/Squaring_the_circle) was an ancient challenge, proven to be impossible with a straightedge and compass in the 1800s. He may have been trying to square the triangle with just those tools.