Most mathematical notation wasn't invented until centuries later. At the time, they would just write calculations out in words. In particular, the use of x representing an unknown quantity was introduced by Descartes: https://en.wikipedia.org/wiki/History_of_algebra#The_symbol_...

For folks who are curious of this, one potential example/discussion of this is Thomas Harriot's _Artis Analyticae Praxis_ (ob. discl., I did some of the typesetting of the Muriel Seltman/Robert Goulding translation)