Even though you haven't bothered to explain it, or to attach a picture,
I'm going to assume that 'R' and 'T' are the ends, and in order to go
from 'R' to 'T', it's necessary to pass through 'S'.
If that's true, then RS + ST = RT .
Knowing that, we can say that (8y + 4) + (4y + 8) = 15y - 9
Remove parentheses: 8y + 4 + 4y + 8 = 15y - 9
Combine terms that are alike: 12y + 12 = 15y - 9
Subtract 12y from each side: 12 = 3y - 9
Add 9 to each side: 21 = 3y
Divide each side by 3 : 7 = y
RS = 8y + 4 = 56 + 4 = 60
ST = 4y + 8 = 28 + 8 = 36
RT = 15y - 9 = 105 - 9 = 96
Check: RS + ST = 60 + 36 = 96 = RT yay !