Is this your question?
= sqrt(7) + i^2 -[6-sqrt(-81)].
= sqrt(7) + [sqrt(-1)]^2 -6 + sqrt(81 * -1)]
= sqrt(7) -1 -6 + sqrt(81)*sqrt(-1)
= sqrt(7) -1 -6 + 9*i
= sqrt(7) -7 +9i.
= -7 + sqrt(7) + 9i.

Hope this answers your question.
For more information, try watching some of the videos here at KA like:
https://www.khanacademy.org/math/algebra/rational-exponents-and-radicals/simplifying-radical-expressions/v/adding-and-simplifying-radicals
https://www.khanacademy.org/math/algebra2/complex-numbers-a2/complex_numbers/v/adding-complex-numbers