$21 I believe (it's 1 am where I am so I'm not 100% sure but I think it's correct)
A = (a+b)/2
2*A = 2*(a+b)/2 ... multiply both sides by 2
2A = a+b
2A-a = a+b-a ... subtract 'a' from both sides to isolate b
2A-a = b
b = 2A-a
Answer: b = 2A-a
We can't answer the question, you haven't given us the information, we need to know what the x is here, or if they gave you a value.
We are given that triangle AOB