Re: this may be positive that is ...
It's a little difficult because he has his back turned but it sounds like he says "that is this line of S" and then he says it again. I think he is pointing at two different parts of the blackboard when he says the two phrases. Perhaps you need to look through the preceding part (Lec 15) to see what he wrote on the right-hand end of the blackboard. If it looks like "a line of S", then it will make sense (grammatically at least!)
Originally Posted by anhnha
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.