Answer:
The answer is 56.
Step-by-step explanation:
According to the text, 21 corresponds to 3 (the side lengths). The relationship between them is 7 (3x7=21). Now, 3 was a side length of the second triangle, like 8 is. To find the missing side of the first triangle, do 8x7, which will give you 56.
Hope that helps!