first you need to save the matrix to a variable name
click second then matrix
go to edit (2 moves to the right) and hit enter to edit the A matrix
type in the size of the matrix then press enter
enter the numbers in the matrix, then press 2nd then quit
now you are back to the main screen. If you want to do something with your A matrix, perhaps put it in reduced row echelon form do this:
press 2nd matrix, then scroll right to "math", then scroll down to rref (you will have to scroll down some distance to find it), then press enter
then press 2nd matrix, and choose the matrix you want (you will probably want the A matrix and it will start on it) then just press enter
now it should say:
rrow([A]
to make that statement complete you must manually close the paranthesis, just press the ")" key, then press enter and it will display your matrix in RREF.