The 20 represents the specific section, the 8 adds onto that side making the full side 8.
-> See attached
Using this, we can set up a proportion to solve;
Sides of the smaller triangle are on the top, and sides of the larger triangle are on the bottom
[Proportion we made]
[Cross-multiply]
20x = 140
[Divide both sides of the equation by 20]
x = 7 ✓
This is how they reached 7, I hope this answer explains it so you can understand.
Have a nice day!
I hope this is what you are looking for, but if not - comment! I will edit and update my answer accordingly.
- Heather