box% sage
┌────────────────────────────────────────────────────────────────────┐
│ SageMath version 9.1, Release Date: 2020-05-20                     │
│ Using Python 3.8.6. Type "help()" for help.                        │
└────────────────────────────────────────────────────────────────────┘
sage: R.<x> = QuadraticField(7)
sage: A = matrix([[4+2*x,6+x,-x],[x,x,-5+10*x],[1+x,-x,2*x]])
sage: A
[ 2*x + 4    x + 6       -x]
[       x        x 10*x - 5]
[   x + 1       -x      2*x]
sage: A.determinant()
243*x + 614
sage: x^2
7
sage: