since S is a point on the line RT, therefore:
RS+ST = RT
5y+5+3y+7 = 12y-20
separate the variables and solve to get thevalue of y as follows:
5+7+20 = 12y-3y-5y
32=4y
y = 8
substitute in the equation of each line to get its length as follows:
RS = 5y+5 = 5(8)+5 = 45
ST = 3y+7 = 3(8)+7 = 31
RT = 12y-20 = 12(8)-20 = 76