In this video, it sounds as if the lecturer is saying:
...sub this straight keen
A few seconds later, he then says
...so we're taking our function and
subbingit in 3 X plus Delta X...
Now I know that "sub" is an abbreviation of "substitute". Also, the online Cambridge English dictionary gives this explanation of sub
in sports, a player who is used for part of a game instead of another
So the lecturer's second use makes sense (at least to me). But my question is about his first use. There, is he actually trying to say "sub this straight in" (and not "sub this straight keen")?