How did he come to the b side of the column in the beginning of the back substitution step?

Question

anonymous · Answer

I am speaking of the 2nd video at 16:11

anonymous · Answer

figured it out, repeat the first 3 steps for column b!

anonymous · Answer

i am doing the Project in cognitive radio in my M.E. course in cognitive radio. Now my mathematical formulation 
$$convex(Ps, Pn)=(\sum_{n=i}^{N}(Rth \div GsPs) )+ (Rth \div GsPs) +\mu1(Ps+\sum_{n=1}^{N} Ps +Pt) +\lambda1(Psh1-Ty)+\lambda2(\sum_{n=1}^{N}Psh2-Tx)$$

anonymous · Answer

Rth, Gs,Gn,Tx.Ty ,Pt are constant ,h1 and h2 are not constant , they are the channel gain. Ps,Pn are the source and relay power of cognitive radio .  how i can find Optimum value of souce(Ps) and Relay (Pn) node of cognitive radio using kkt condition