Hi there!
Since triangle RST is stated with the sides in order (R is connected to S, which is connected to T, which is then connected to R), the side between R and S is T.
Hope this helps!
Step-by-step explanation:
4x
__________
| |
x-3| | x-3
| |
|__________
4x
Add all sides together to get perimeter
2(4x)+2(x-3)=10x-6
Ax + By = C
3x + 2y = 12
A=3, B = 2 and C = 12