if z1 = 3 cis pi then we can expand this to 3 (cos pi + i sin pi) which is also equal to 3 ( -1 +0i) which equals -3i and we multiply this with 5 ( cos pi/2 + i sin pi/2) or 5 ( 0 + i 1) or 5i
-3i * 5i = -15i
In a + bi form it would look like 0 + (-15i)
so a = 0 and b = -15
