Solution 
Switch sides so that the right side is the left side 
1/2h (B + b) = A 
Now multiply both sides by 2 
2 • 1/2h (B + b) = A • 2 
Simplify the equation 
h (B + b) = 2A 
Now divide both sides by h
(h (B + b))/h = 2A/h 
Simplify the equation 
B + b = 2A/h 
Subtract B from both sides 
B + b - B = 2A/h - B 
Simplify the equation
b = 2A/h - B 
Therefore b = 2A/h - B