Substraction in $\mathbb {Z}$/p$\mathbb {Z}$ or in $\mathbb {Z}$/p$\mathbb {Z}$[x] : -

- substracts two integers in $\mathbb {Z}$/p$\mathbb {Z}$ or two polynomials in $\mathbb {Z}$/p$\mathbb {Z}$[x]. For polynomial expressions, use the normal command to simplify.
For integers in $\mathbb {Z}$/p$\mathbb {Z}$, input :

31%13-10%13

Output :

-5%13

For polynomials with coefficients in $\mathbb {Z}$/p$\mathbb {Z}$, input :

normal((11*x+5)%13-(8*x+6)%13)

or also :

normal(11% 13*x+5%13-8% 13*x+6%13)

Output :

(3%13)*x+-1%13