Question 14339
Well, in order to solve for A, you need all the A's on one side. so, you move the ac over with the 2a. 
{{{2a-ac=3b+b}}}
Don't forget to change the signs when moving across the equal sign.
{{{2a-ac=4b}}}
Factor out the A 
{{{a(2-c)=4b}}}
Divide both sides by {{{(2-c)}}} to isolate the A
{{{a=4b/(2-c)}}}
That should be your answer in terms of A
Hope this helps!