Solve the literal equation for the given variable.
a= bh/2 (bh over 2)
solve for b. with solution .

If I understood your question right, he right equation would be b=2A/h

If A is on the one side of the equation, and bh/2 is on the other side, it means that our goal is to get to the point when b is alone on one side of the equation, doesn't matter which. 

When working with equations like this, always remember that if X is the number that you divide by on one side, when you transfer it to the other side of the 'equal' sign it is going to switch is function and become a multiplier. So, in this case, when we transfer the number two on the other side it is going to become 2xA and we will have left 2 x a=b x h.

To get b alone, as we wanted to, we will transfer h on the other side and it will become the divider, adn we will get the solution: 2A/h=b 

Hope this helps :)