Compute a - 6i^3 and (6i^3)a, where a .
a. a - 6i^3 = a + 6i, (6i^3)a = -6ia
b. a - 6i^3 = a - 6i, (6i^3)a = -6ia
c. a - 6i^3 = a + 6i, (6i^3)a = 6ia
d. a - 6i^3 = a - 6i, (6i^3)a = 6ia