There's more than one way in which to check this answer thru estimation. I'd find 6*60 and 6*70 separately, obtaining 360 and 420. It's obvious that Kendra's "390" is between 360 and 420, so it's likely that she's right.
In order to estimate the product of 6x65 in my head without writing anything down on paper or running home for my calculator, I close my eyes and I see something like this floating in space in front of me:
[6 · cos(2π)] · e⁰ · [60 · (1Hz/1cps)] = 360 but 65 is a little more than 60 so the product is probably a little more than 360 so 390 could be pretty close.
