Yes you have the right idea. You find the derivative to get the velocity function and then determine when the velocity is zero. That turns out to be at t = 2 and t = 6. Both are correct. The final answer is correct.
The only flaw I see is that you wrote x'(t) = 3y^3 - 24t + 36 on line 2 when it should be x'(t) = 3t^2 - 24t + 36