Let that be a lesson to me, always watch the video.
Many appologies Byrdie, I watched the video and now see what your talking about, my bad.
I still don't think you have a problem, though. At the end of the video, he's figuring the power of ten by which the answer is multiplyied, and does a quick look at his notes to avoid the simple math -34 + 8 + 7 and reads the answer as -21. That seems to me to be in error, as you also said, the result should be 3.9769 * 10 ^ (-19)