Okay, not complete so I had to add some things like, say, output. But it does appear to work.

Starting with: 3(7x-1)+10x-4+3x=90x+1 it outputs:
3 7 x * 1 - * 10 x * + 4 - 3 x * + 90 x * 1 + -
[download]

So .. what will you do with this? What was the point of getting it into RPN in the first place?

I was thinking that putting an expression into RPN would be a good idea since it eliminated all () and the expression could be thought of like a tree