Identify an equation that describes some chemical process.  Your equation should have at least two parameters, at least one of which should contribute in a non-linear way.

Here is an example of such an equation.  This describes the spectroscopic signal during a pH titration.

y = (A-B)*(10^(pH-pK) / (1 + 10^(pH-pK) )) + B

 where

 A: the signal from the protonated form

 B: the signal from the ionized form

pK: The pK of the reaction

pH:The independent variable

 y: The dependent variable (measured signal).

y is linear in A and B, but non-linear with respect to pK

You can use this equation, but you will lose two points (out of 20) if you use it instead of some other equation