You can put this solution on YOUR website! Isolate the r
ar-3r=-6t-2
r(a-3)=-6t-2
r=-(6t+2))/(a-3)
I missed the ar at the beginning.
let r=2 and a =5
10+2=6-6t
12=-6t
6=-6t
t= -1
4/2=2 for r
Collect the terms containing "r" in the left side.
Collect all other terms in the right side.
When you move some term from one side to the other, do not forget to change its sign.