Suppose S is between R and T. Use the segment addition postulate to solve for each variable. RS = 7y-4, ST = y+5, RT = 28

RS+ST=RT

RS=7y-4

ST=y+5 using RS and RT in the first equation gives you:

7y-4+y+5=RT combine like terms on left side

8y+1=RT, and we are told that RT=28 so:

8y+1=28 subtract 1 from both sides

8y=27 divide both sides by 8

y=27/8

(RS=19 5/8, ST=8 3/8, RT=28)