Answer:
b
Explanation:
A. you can just sum the two terms containing a "b"
3b + 10b = 13b
B. can be writen like 3 × b × 10 × b
3b. 10b = 3×b×10×b = 30×b×b = 30b²
c. same as in A.
9b +21b = 30b
D. is any of those numbers a power? if not, it's the same as in B.
9b21b = 9×b×21×b = 189b²