I believe your first answer (for problem 3) is correct, however, I think the correct answer for problem 4 should be 3 - 4i. i is the same thing as √-1, and whenever it's within a solution, you need to consider the conjugate, meaning the positive and negative values for it. So, if you have 3 + 4i, 3 - 4i should also be a solution.
Hope this helped!! xx
