The object is to get everything except for "h" on one side of the equation.
The equation says that everything on the right side is being multiplied by 1/2.
What do we have to multiply times 1/2 to get 1? The answer is 2.
So multiply both sides by 2 to get rid of the fraction.
2A = (a+b)h
Now we are multiplying h times (a+b) so to get rid of (a+b) we have to divide both sides by that term.
(a+b)/(a+b) is 1
So you are left with: 2A/(a+b) = h
Done!