(a + 4i)(9 - bi) Substitute a = 6
(6 + 4i)(9 - bi) Remove the brackets with a foil.
54 - 6*bi + 36i - 4bi^2 Simplify
54 - 6bi +36i + 4b
The only way this can be a real number is if the is disappear. so
-6bi + 36i = 0
-6bi = - 36i Divide by -6i
b = 6
C <<<< answer