In A + A(B) = C, the parenthesis around (B) is unnecessary and so confusing, it is A+AB=C... Also you can use wolframalpha that's more cool than GFY offtopics:
http://www.wolframalpha.com/input/?i...AB+%3D+C+for+A
But for the step by step info it asks cash
Fully free it is:
http://www.sagemath.org/