Both in school and university, systems of simultaneous equations were denoted by a straight line in front of the equations (like a determinant, but no line after).
Now I find out that both LaTeX and all google results look like this:
What are the conventions?
I live in Bulgaria, Europe. Here are a couple of examples from one book of analytic geometry, another, independent one uses the same notation.
I have always seen the notation $$ \begin{cases} &x_1 = 2r+s-t \\ &x_2 = r \\ &x_3 = -2s + 2t \\ &x_4 = s \\ &x_5 = t \\ \end{cases} $$ I'm fairly sure this is standard notation, but you can of course adapt your notation as long as the reader understands (clarification might be needed).