If you would like to simplify 7 - 3[(n^3 + 8n) / (-n) + 9n^2], you can do this using the following steps:
7 - 3[(n^3 + 8n) / (-n) + 9n^2] = 7 - 3[(-n^2 - 8) + 9n^2] = 7 - 3[-n^2 - 8 + 9n^2] = 7 - 3[ - 8 + 8n^2] = 7 - 3[8n^2 - 8] = 7 - 24n^2 + 24 = - 24n^2 + 31
The correct result would be - 24n^2 + 31.