A=1/2H(B+b) Solve for H
Let's start by getting rid of the fraction.
Multiply both sides by 2 (whatever operation you do to one side of an equation you must do to the other side as well):
2(A) = 2[1/2H(B+b)]
2A = H(B+b)
Now divide both sides by (B+b) and end up with this:
2A/(B+b) = H
I hope this will help.