Let's write out my matrix a-- my matrix a, a[1,1] a[1,2] all the way to a[1,n] and then this, as we go down a column, we go all the way down to a[m,1] and then as we go all the way to the bottom right, we go to a[m,n] and I'm going to multiply that times the 0 vector that has exactly n components.