Each article has their own discussion section. This way you can easily see if other students have had the same problem as you and asked about it already.
Figure 28. Click the "Discussions" button to see if other people hit the same problem as you and created a discussion thread for it. We see that there are 3 total discussions about this header, so let's check them out. URL: ourbigbook.com/barack-obama/mathematics#fundamental-theorem-of-calculus
Figure 29. From these 3 recent threads, the first one has one like, and it seems that someone found the same difficulty that we did. URL: ourbigbook.com/go/issues/barack-obama/fundamental-theorem-of-calculus
Figure 30. So let's inspect that discussion. Ah, clearly, the comments are very illuminating, our problem is solved! URL: ourbigbook.com/go/issue/1/barack-obama/fundamental-theorem-of-calculus