Answer:
B = 24
Explanation:
we just need to work out the pattern the given numbers are following
from 9 to 12 is an addition on +3, likewise from 12 to 15 is an addition of +3
thus the following value will be 15 + 3 = 18 when we add 18 + 3 = 21
to get B we add 3 to 21
21 + 3 = 24