1. The problem statement, all variables and given/known data
2. Relevant equations
3. The attempt at a solution
||B|| = ||M_1 * A * M_2 ||
So from an equality following from the norm, we can get...
||B|| <= ||M_1||*||A||*||M_2||.
Now, we know that B is a submatrix of A. So if A is 4x3, then M_1 must be 1x4 and M_2 must be 3X1 (I know that block matrices are more complicated than that, but this might work). What this also means is that the combined product of M_1 and M_2 must be <= 1. But beyond that, I'm stuck. Is there another step I should take?