For a simple linear equation with an arbitrarily complex input stream, e.g. 2+3x+7(x+27)-6 you can probably do it with a minimum of conditionals. The hard part is writing a parser, but it's also entertaining. How I might do it, based on not spending much time on it and not having done one in a while is:

a) Write a parser that splits on the equal sign and multiplies everything out to eliminate parentheses, so my expression above would be 2+3x+7x+189-6.

b) do a second pass where you look at each term and add it into a pair of accumulators, one for the constants and one for the slopes. When you process the righthand side, multiply everything by -1 before you add it.

Now you have something like 3235643596 in the slope accumulator (corresponding to the "x" terms) and 998396 in the constant accumulator. Do a simple division of one by the other and you're done.

Writing the parser is the hard part, and it shouldn't be that hard for your case (you should be able to do it with a little recursive chunk of code)