(Note on the wikipedia article, start about halfway down, where it says "Explicit formulas for small systems".)
Also, and this gets closer to the point, drilling by solving 3 x 3 matrices is not really about the rule, which is pretty simple. It's about making an easy problem harder by stuffing a lot of arithmetic into it. And to me, that is symptomatic of a big difficulty with math education as I'm encountering it through my son. Computers are much better at computing sums than humans. It's just silly to drill students on arithmetic. If you want to do something complicated, why not derive Cramer's rule?
So, I decided to write a solver for 3 x 3 systems in Python. I wouldn't say it's thoroughly debugged yet, so let me know if you run into a problem. With the example shown, I did get the same answer as this online calculator.
The first code segment contains the equations explicitly entered as an array. I'm sure you know how to modify it to read input from a file.
The output looks like this: