2m^2-5m-3 can be written 2m^2-6m+m-3. 2m is a common factor for 2m^2 and 6m, i.e., 2m(m-3). That leaves m-3, which happens to be what's in the brackets. So the whole expression becomes (2m+1)(m-3). The original question was 4m^2-5m-3, but this doesn't factorise. We would have to write the original question: 2m^2+(2m+1)(m-3).