You have two problems, both of which arise from not paying attention to multiples of n. First, you have 6n + 5 = -10t. If you subtract 5 from both sides, you get 6n = -10t - 5. Second, the exponent is not n+2, but 3n+2. If n = (-10t - 5) / 6, then 3n + 2 = 3[(-10t - 5) / 6] + 2.