So we get minus two plus two or zero for the new length C prime B for the edge A comma B. Similarly for the edge B, C, we take the original length -one, we add to it -two, and then we subtract -three but as we add three, then again it all cancels out, we get zero for the new cost of arc B, C. For the arc C comma A we take the original length four, we add minus three, we subtract zero. 