Your answer is c. As you multiply 1/2 by both of the factors inside the parentheses you’ll find that 1/2(b-30)=1/2b-15
Sometimes it helps to draw it out...
(17)
C_______M_________Q
|___________________|
21
therefore, CM + MQ = CQ
17 + MQ = 21
MQ = 21 - 17
MQ = 4 <===
the first answer is 93000000 and the second answer is 240000