If you run the code, you will get the error, as m is undefined, as neither b==2,b==6 or b==10, b value from the code is vector 30x1, not scalarsingle value. These if condition are not satisfied in any three cases, therefore m value is not assigned, so its reflects the error m is undefined.
If you manage to run the if condition anyhow, then m value assigned as one scalar value.
Due to different sizes of a,b,l,m, it cannot concanated. Where a,b,l are 30x1, whereas m will a scalar value.