Changed this a little, as I'd missed the horizontal line).
Create a new equation, then copy/paste the following:
\matrix(+&\matrix(a&b@c&d))/\matrix(b&c&a)
and press Enter at the end.
The \matrix keywords will transform into \'square black blob character" at some point each "&" introduces a new column, and each "@" introduces a new row). Math autocorrect needs to be "on" to make it work - it should be "on" by default.
It should look like this:
For more space over the line, you can insert a blank line in the upper matrix, like this:
\matrix(+&\matrix(a&b@c&d)@ & )/\matrix(b&c&a)