R(s + t) = st
Rs + Rt = st
Rs - st = -Rt
s ( R - t) = -Rt
*s = - Rt / (R - t) Multiply through by -1 on the right.
*s = Rt / (t - R) Put in the brackets in your answer. What you have been writing means
s = Rt/r - R you need the brackets. The stars mean that either could be an answer. I think your problem could be the brackets. Now I'm reasonably certain this is correct.