A-Best: To get the conjugate, just flip the sign on the imaginary parts' coefficient.
=> -4 - 6i
i's repeat every four times. i° = 1, i^(1) = i, i² = -1, i³ = -i, ..... and repeats in this pattern. therefore, if you just follow the pattern and you will find that
i^(89) = i² => -1
same rule apply for the other expression:
16i^(64) + 19i^(50) => 16(i³) + 19(i³) = -16i - 19i = -35i
