Answer:
9
Step-by-step explanation:
multiply both sides by 15 to get:
45/5 = x
9 = x
ST = 20
RT is the sum of RS and ST
Replacing with length you get:
17 + x + 6 = 3x - 56
17 + 6 + 5 = 3x - x
28 = 2x
14 = x
ST = x + 6 = 14 + 6 = 20