Hello Mike! This is thunderlord.
First of all I am sorry to bring this question up again because it has been brought up 2 times already. But I am curious on why E could not be the answer because RS and ST could both equal 9 as the triangle is isosceles triangle. Moreover, RS+ST>RT or 9+9>ST. So can’t anything above (the perimeter) 35 a wrong answer? (pg 152 number 18). Thank you so much for reading!

You’re only told that one side of an isosceles triangle is 9, not necessarily that two sides are 9. If the sides are 30, 30, 9, then you can have a perimeter of 69 and no problems with the triangle inequality rule.