The universal answer to your question is:
"Plug the values back into the equations and see if they make sense"
What happens when you plug the values back into your equations? take slopeEqn for example:
The values are very close. Since you're using NSolve, you'll only get approximately correct answers because of the nature of floating point numbers.
You haven't defined "f" or "g" for us, so there's nothing I can say about the first equation.