If you click on the left or right screen button, nothing happens. (You CAN, however, move to the next/previous card by clicking right below one of the buttons or by using the right/left arrow key.)
The problem: the card's text fields partially overlap the arrow buttons at the bottom. (These fields are invisible, so you can't see them if you're browsing.) When you click on the buttons, you click in the text fields instead.
Here's how to repair the card:
1. Put HyperCard into Edit mode.
2. Bring up the "Math Functions (cont)" card.
3. Select the Field mode in the Tools menu (the upper-right box). When you do this, the text fields become visible, and you'll see the overlap.
4. Click the arrow in the lower parts of the three fields that overlap buttons, and move them up.