XFCE does not come with a calculator so I did some research and found Qalculate.
It is a general purpose scientific calculator that has massive functionality.
Expressions may contain any combination of numbers, functions, units, variables, vectors and matrices.
Supports complex numbers, infinite numbers, sexagesimal numbers, all number bases from 2 to 36, time format and roman numerals.
Does symbolic calculations, factorization, simplification, differentiation, integration, can solve most equations and inequalities.
Has many builtin functions -> math, financial, statistical, etc.
Does unit and currency conversion plus can retrieve current currency rates from the Internet.
It has many builtin common constants like pi, e, Avogadro's number, etc.
Can plot functions or data using Gnuplot.
Displays whether result is exact or approximate.
Has a calculation history. This is a fantastic feature. I can't tell you the number of times I do several calculations and want an earlier one but did not write it down. With this you just click on history and scroll through all previous calculations.
The entropy name is: qalculate-gtk