Is there a phrase sounds like “sub this straight keen”?

Question

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 subbing it 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")?

Answer 1

the lecturer is saying "sub this straight in"

Yes, that's exactly right. To be honest I'm not sure why you are hearing it as "keen"; he seems perfectly clear to me. I'm not hearing any sign of a "k" at the start of his pronunciation of "in", and the vowel portion sounds like a perfectly good "ih", with little or no hint of "ee". But I guess it depends on how familiar one is with spoken English.