Question

Solve for b A= \frac{a+b}{2}

Answer 1

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