Me explain do not need work just explain you i need

Alright, 

you will need to solve for each variable to find if they got the equation correctly or not.. 

Let's start with h 
isolate h 
(1/2)h = A/(b1+b2)
Now we need to get rid of 1/2 we may do so by multiplying both sides by 2 

h = 2(A/(b1+b2))

And that's not how they did it for h so Option D doesn't apply 

Let's see C
Also doesn't apply since they multiplied 2 only by A rather than A/h

Let's see B
Also not correct they made the same mistake as with option C

Let's see A
again same mistake 
when multiplying both sides by 2 
the 2 should be as following 


I believe you should contact your instructor and explain that non of the options is right. 

Your answer should be “b” i truly hope i could ! good luck (: