You know that AB = RT. You also know that AB = 45, and that RT = 3(x+9). Therefore, you can take the following steps.
AB = RT
Use substitution to get 45 = 3(x+9)
Divide both sides by 3 to get 15 = x + 9
Subtract 9 from both sides to get x = 6
That is your answer. I hope it helps!
