Answer:
We can treat i as if it was a variable/symbol, with the caveat that
i * i = -1
[6]
(4 - 13i) - (3 + 6i) = 4 - 13i - 3 - 6i = 4 - 3 - 13i - 6i = 1 - 19i
[7]
-1 + (-7 + 8i) - 18i = -1 - 7 + 8i - 18i = -8 - 10i
[8]
2i * (4 + 3i) = 2i * 4 + 2i * 3i = 8i + 6i^2 = 8i + 6*-1 = -6 + 8i
[9]
(6 - i) * (9 + 3i) = 6 * 9 + 6*3i - i*9 - i*3i = 54 + 18i - 9i - 3i^2 = 54 + 9i - (3*-1) = 54 + 9i + 3 = 57 + 9i
