So simple as it looks exponential equation is "y = axb", in which "a" and "b" are constants.
Most equations are written with spaces between the variables and operands. The use of superscripts is important, too.
It really isn't clear what you are trying to do, with negative values. The Arduino doesn't do negative voltages. The Arduino doesn't do anything but constant PWM values.
Perhaps if you drew a picture of what you are trying to accomplish, with properly labeled X and y axes, it would make more sense.