B prime would not be the same as b. I guess your question is to rearrange the expression making b the subject.
here is the answer so you can check
The problem is:
A=1/2h(b + b'), solve for b.
I would know how to solve it, except for the b'. What is that and how do I solve the problem with it? Is it the same as b? I do not know. If someone could clear me up with this matter, I would be very thankful.