I tried doing it myself but just can seem to understand it.
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
4 X 2