[firedrake] Questions about Firedrake PETSc.

Dear the development team, I am Junho, a researcher at University of Leeds. Using Firedrake PETSc, I am trying to solving a linear system. To do this, I refer to your instruction about Interfacing directly with PETSc(https://www.firedrakeproject.org/petsc-interface.html), but it is hard to follow the instruction for me because of no example. So would you let me know how to apply the instruction if simply setting a system Bx=b where B=[1, 2, 3; 3, 4,5; 4, 5,6], b=[2; 3; 5]? In addition, if I solve a linear system with PETSc like your instruction, can I use parallel computing to shorten computing time? If yes, how can I use the parallel computing? Thank you. Junho