The calculations on the back of the envelope are for working out the lengths of the diagonal spreaders for a 14MHz quad antenna.

I'm a ham and well remember quad antennas. They were inexpensive, some people created them using bamboo for spreaders. Just wire and bamboo with a small boom. What those who tried them in Michigan found out they wouldn't survive a single winter, even when they moved to fiberglass instead of bamboo.

The period of popularity lasted from the late sixties until the early eighties. You will still find them in tropical climates but they are no where near as popular as they once were.