Algebra in Excel; Permitted Disparity and Self-Employed Earned Income

Permitted disparity and self-employment earned income; I do not feel sure about the solution that I came up with?

Permitted disparity and self-employment earned income, to some people, present a Gordian knot. However, it seems that I somehow stumbled onto a relatively easy solution. However, I steel uncertain of the mathematical validity of my results. Does someone feel willing to contact me off the boards and see the file I came up with?

What functions can one use in Excel to solve algebra problems?