Answer:
35.1 units
Step-by-step explanation:
Imagine a right-angled triangle with RS as hypotenuse. The base would be 3 units and the height would be 6 units. By Pythagorus:
RS²=3²+6²
RS=≈6.7 units
Imagine a right-angled triangle with RT as hypotenuse. The base would be 9 units and the height would be 12 units. By Pythagorus:
RT²=9²+12²
RT= = 15 units
Imagine a right-angled triangle with TS as hypotenuse. The base would be 12 units and the height would be 6 units. By Pythagorus:
TS²=12²+6²
TS=≈13.4 units
Perimeter=RS+RT+TS
=6.7+15+13.4
=35.1 units
My father taught me a method that I call, 'throwing some thing over the wall'. There is a sample equation for the first picture that shows you how it works. It's a great way to help isolate the variable and I hope that it works for you. The second picture contains the answers I got through the use of this method. I'm sorry for the messy handwriting; I was on a bus.