Help me fix my calculator

Hi, I recently made a calculator. It works perfectly as far as I'm aware. (it's called Calculator Logic.vi and it's attached below).
My teacher saw it and asked me to add parentheses.
So I started working on a component that breaks down any expression into a number of "parenthesized expressions", and solves every one of them individually. This component is a key step in adding parentheses functionality to my calculator.

Now the problem is: The calculator solves the parenthesized expressions incorrectly! It solves the first one correctly, and then it's clear it keeps past values in memory and they mess up the next calculations! I couldn't find the source of it.
I know it's probably a little complicated to fix, but you guys got a better chance at it than I do.