Convert from cuerda to X unit

Unit Definition (cuerda)
The Cuerda is a traditional unit of distance in Guatemala, equal to 25 varas or about 21 meters (roughly 69 feet). Since cuerda means a cord or rope in Spanish, this unit probably arose as the length of a measuring rope. The cuerda is also used as an area measure equal to 1 square cuerda or 625 square varas; this is about 440 square meters or 527 square yards. Note that the cuerda is also a traditional unit of land area in Puerto Rico, as well as a volume measurement for firewood in Cuba.

Unit Definition (X unit)
The X-unit (symbol xu) is a unit of length formerly used to measure the wavelength of X-rays and gamma rays. The X-unit is approximately 1.0021×10-13 metre. Defined by the Swedish physicist Karl Manne Georg Siegbahn (1886-1978) in 1925, the X-unit could not at that time be measured directly; the definition was instead made in terms of the spacing between planes of the calcite crystals used in the measuring apparatus.