Last 2 i need help on , very confused

As stated in my other answer, the first one can't be determined.....we can compute  R1 - (1/2)R2    but that doesn't tell us WHAT  to do with the result......

The second  is easy.......a matrix's  dimensions are expressed as  Rows X  Columns

Note that we have   4 Rows and 2 Columns

So

4 X 2