Question 641213
To solve equation for b

{{{ A=1/2(a+b)h}}}
First multiply both sides by 1/2
{{{2A=(a+b)h}}}
Then divide by h
{{{2A/h=a+b}}}
Lastly, subtract a
{{{2A/h-a=b}}}
To give the function for b. 

Voila!