I think the first photo, to find the answer, you have to add two sides together. If they are greater than the third, it’s a triangle. If not, then it’s not a triangle.
For A on the slide with 4 cards, the two angles labeled x are equal to each other. A triangle has 180 degrees. Subtract the given angle from 180 and divide by two.
I think those clues can Apple to most of the other pictures. That’s the most I can remember from my geometry class.