For this question you have to solve the equation backwards.
A = 1/2 x (b1 + b2) x h
In order to get rid of the 1/2 (since it's division) multiply each side by 2
now we have: 2A = (b1 + b2) x h
Now do the same to the get rid of the (b1 + b2) (divide each side by it)
2A/(b1 + b2) = h
